Uninitialised memory use when Dumping undef'd string

**Description**

Uninitialised memory use when dumping an undef'd string.

Reported by damien on #p5p.

**Steps to Reproduce**

```perl
use Devel::Peek;

open my $fh, '>', \my $str;
print $fh "xxxxx";
undef $str;
print $fh "y";
Dump($str);
```

```shell
$ valgrind --track-origins=yes -q perl a.pl
SV = PV(0x508cea0) at 0x50c77a8
  REFCNT = 2
  FLAGS = (POK,pPOK)
  PV = 0x518e880 ==2119893== Conditional jump or move depends on uninitialised value(s)
==2119893==    at 0x4927944: Perl_pv_escape (dump.c:243)
==2119893==    by 0x4928C9E: Perl_pv_pretty (dump.c:400)
==2119893==    by 0x4928C9E: _pv_display_flags.lto_priv.0 (dump.c:424)
==2119893==    by 0x4934903: Perl_do_sv_dump (dump.c:2216)
==2119893==    by 0x487B722: S_do_dump (Peek.xs:334)
==2119893==    by 0x487B7FD: S_pp_dump (Peek.xs:347)
==2119893==    by 0x4A8DFD2: Perl_runops_standard (run.c:41)
==2119893==    by 0x48F8E9B: S_run_body (perl.c:2884)
==2119893==    by 0x48F8E9B: perl_run (perl.c:2799)
==2119893==    by 0x40005B2: main (perlmain.c:127)
==2119893==  Uninitialised value was created by a heap allocation
==2119893==    at 0x484AB04: malloc (vg_replace_malloc.c:447)
==2119893==    by 0x4A96147: Perl_safesysmalloc (util.c:176)
==2119893==    by 0x4A96147: Perl_sv_grow (sv.c:1428)
==2119893==    by 0x49906C3: PerlIOScalar_write (perlio.c:1295)
==2119893==    by 0x4922F8E: Perl_PerlIO_write (perlio.c:2010)
==2119893==    by 0x4922F8E: Perl_do_print (doio.c:2254)
==2119893==    by 0x49D8DA8: Perl_pp_print (pp_hot.c:2148)
==2119893==    by 0x4A8DFD2: Perl_runops_standard (run.c:41)
==2119893==    by 0x48F8E9B: S_run_body (perl.c:2884)
==2119893==    by 0x48F8E9B: perl_run (perl.c:2799)
==2119893==    by 0x40005B2: main (perlmain.c:127)
[...]
```


**Expected behavior**

No uninitialised memory use and clean Valgrind results.

**Perl configuration**

```
$ perl -V
Summary of my perl5 (revision 5 version 42 subversion 0) configuration:
   
  Platform:
    osname=linux
    osvers=6.17.12
    archname=x86_64-linux
    uname='linux localhost 6.17.12 #1 smp preempt_dynamic fri dec 12 23:20:10 gmt 2025 x86_64 amd ryzen 9 3950x 16-core processor authenticamd gnulinux '
    config_args='-des -Dinstallprefix=/usr -Dinstallusrbinperl=n -Ui_xlocale -Ui_ndbm -Di_gdbm -Ui_db -DDEBUGGING=-g -Dlibpth=/usr/local/lib64 /lib64 /usr/lib64 -Dnoextensions=ODBM_File -Duseshrplib -Darchname=x86_64-linux -Dar=x86_64-pc-linux-gnu-ar -Dcc=x86_64-pc-linux-gnu-gcc -Dcpp=x86_64-pc-linux-gnu-gcc -E -Dld=x86_64-pc-linux-gnu-gcc -Dnm=x86_64-pc-linux-gnu-nm -Dranlib=x86_64-pc-linux-gnu-ranlib -Accflags=-O3 -march=native -flto=jobserver -fno-semantic-interposition -mtls-dialect=gnu2 -g1 -pipe -fcf-protection=none -fdiagnostics-color=always -fdiagnostics-urls=never -frecord-gcc-switches -fdiagnostics-show-context=3 -Wa,-O2 -Wa,-mtune=znver2 -Warray-bounds --param=ggc-min-expand=120 -Wstrict-aliasing -Wfree-nonheap-object -Wuninitialized -Wmaybe-uninitialized -Wreturn-type -Wbuiltin-declaration-mismatch -Wtautological-compare -Werror=lto-type-mismatch -Werror=strict-aliasing -Werror=odr -Warray-bounds -Wstringop-overread -Wstringop-overflow -Wstringop-truncation -Wformat-overflow -Wformat-truncation -Wrestrict -ggdb3 -Wformat -Werror=format-security -fno-strict-aliasing -DNO_PERL_RAND_SEED -Doptimize=-O3 -march=native -flto=jobserver -fno-semantic-interposition -mtls-dialect=gnu2 -g1 -pipe -fcf-protection=none -fdiagnostics-color=always -fdiagnostics-urls=never -frecord-gcc-switches -fdiagnostics-show-context=3 -Wa,-O2 -Wa,-mtune=znver2 -Warray-bounds --param=ggc-min-expand=120 -Wstrict-aliasing -Wfree-nonheap-object -Wuninitialized -Wmaybe-uninitialized -Wreturn-type -Wbuiltin-declaration-mismatch -Wtautological-compare -Werror=lto-type-mismatch -Werror=strict-aliasing -Werror=odr -Warray-bounds -Wstringop-overread -Wstringop-overflow -Wstringop-truncation -Wformat-overflow -Wformat-truncation -Wrestrict -ggdb3 -Wformat -Werror=format-security -fno-strict-aliasing -Dldflags=-O3 -march=native -flto=jobserver -fno-semantic-interposition -mtls-dialect=gnu2 -g1 -pipe -fcf-protection=none -fdiagnostics-color=always -fdiagnostics-urls=never -frecord-gcc-switches -fdiagnostics-show-context=3 -Wa,-O2 -Wa,-mtune=znver2 -Warray-bounds --param=ggc-min-expand=120 -Werror=lto-type-mismatch -Werror=strict-aliasing -Werror=odr -Warray-bounds -Wstringop-overread -Wstringop-overflow -Wstringop-truncation -Wformat-overflow -Wformat-truncation -Wrestrict -Wl,-O1 -Wl,--as-needed -Wl,-z,pack-relative-relocs -flto=jobserver -Wl,--defsym=__gentoo_check_ldflags__=0 -ggdb3 -Dprefix=/usr -Dsiteprefix=/usr/local -Dvendorprefix=/usr -Dscriptdir=/usr/bin -Dprivlib=/usr/lib64/perl5/5.42 -Darchlib=/usr/lib64/perl5/5.42/x86_64-linux -Dsitelib=/usr/local/lib64/perl5/5.42 -Dsitearch=/usr/local/lib64/perl5/5.42/x86_64-linux -Dvendorlib=/usr/lib64/perl5/vendor_perl/5.42 -Dvendorarch=/usr/lib64/perl5/vendor_perl/5.42/x86_64-linux -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Dvendorman1dir=/usr/share/man/man1 -Dvendorman3dir=/usr/share/man/man3 -Dman1ext=1 -Dman3ext=3pm -Dlibperl=libperl.so.5.42.0 -Dlocincpth=/usr/include  -Dglibpth=/lib64 /usr/lib64  -Duselargefiles -Duse64bitint -Dd_semctl_semun -Dcf_by=Gentoo -Dmyhostname=localhost -Dperladmin=root@localhost -Ud_csh -Dsh=/bin/sh -Dtargetsh=/bin/sh -Uusenm'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=undef
    usemultiplicity=undef
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
  Compiler:
    cc='x86_64-pc-linux-gnu-gcc'
    ccflags ='-O3 -march=native -flto=jobserver -fno-semantic-interposition -mtls-dialect=gnu2 -g1 -pipe -fcf-protection=none -fdiagnostics-color=always -fdiagnostics-urls=never -frecord-gcc-switches -fdiagnostics-show-context=3 -Wa,-O2 -Wa,-mtune=znver2 -Warray-bounds --param=ggc-min-expand=120 -Wstrict-aliasing -Wfree-nonheap-object -Wuninitialized -Wmaybe-uninitialized -Wreturn-type -Wbuiltin-declaration-mismatch -Wtautological-compare -Werror=lto-type-mismatch -Werror=strict-aliasing -Werror=odr -Warray-bounds -Wstringop-overread -Wstringop-overflow -Wstringop-truncation -Wformat-overflow -Wformat-truncation -Wrestrict -ggdb3 -Wformat -Werror=format-security -fno-strict-aliasing -DNO_PERL_RAND_SEED -fwrapv -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    optimize='-O3 -march=native -flto=jobserver -fno-semantic-interposition -mtls-dialect=gnu2 -g1 -pipe -fcf-protection=none -fdiagnostics-color=always -fdiagnostics-urls=never -frecord-gcc-switches -fdiagnostics-show-context=3 -Wa,-O2 -Wa,-mtune=znver2 -Warray-bounds --param=ggc-min-expand=120 -Wstrict-aliasing -Wfree-nonheap-object -Wuninitialized -Wmaybe-uninitialized -Wreturn-type -Wbuiltin-declaration-mismatch -Wtautological-compare -Werror=lto-type-mismatch -Werror=strict-aliasing -Werror=odr -Warray-bounds -Wstringop-overread -Wstringop-overflow -Wstringop-truncation -Wformat-overflow -Wformat-truncation -Wrestrict -ggdb3 -Wformat -Werror=format-security -fno-strict-aliasing'
    cppflags='-O3 -march=native -flto=jobserver -fno-semantic-interposition -mtls-dialect=gnu2 -g1 -pipe -fcf-protection=none -fdiagnostics-color=always -fdiagnostics-urls=never -frecord-gcc-switches -fdiagnostics-show-context=3 -Wa,-O2 -Wa,-mtune=znver2 -Warray-bounds --param=ggc-min-expand=120 -Wstrict-aliasing -Wfree-nonheap-object -Wuninitialized -Wmaybe-uninitialized -Wreturn-type -Wbuiltin-declaration-mismatch -Wtautological-compare -Werror=lto-type-mismatch -Werror=strict-aliasing -Werror=odr -Warray-bounds -Wstringop-overread -Wstringop-overflow -Wstringop-truncation -Wformat-overflow -Wformat-truncation -Wrestrict -ggdb3 -Wformat -Werror=format-security -fno-strict-aliasing -DNO_PERL_RAND_SEED -fwrapv'
    ccversion=''
    gccversion='16.0.0 20251213 (experimental) e9c12ffd710484751428fc301a272e16f641cd56'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=16
    longdblkind=3
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='x86_64-pc-linux-gnu-gcc'
    ldflags ='-O3 -march=native -flto=jobserver -fno-semantic-interposition -mtls-dialect=gnu2 -g1 -pipe -fcf-protection=none -fdiagnostics-color=always -fdiagnostics-urls=never -frecord-gcc-switches -fdiagnostics-show-context=3 -Wa,-O2 -Wa,-mtune=znver2 -Warray-bounds --param=ggc-min-expand=120 -Werror=lto-type-mismatch -Werror=strict-aliasing -Werror=odr -Warray-bounds -Wstringop-overread -Wstringop-overflow -Wstringop-truncation -Wformat-overflow -Wformat-truncation -Wrestrict -Wl,-O1 -Wl,--as-needed -Wl,-z,pack-relative-relocs -flto=jobserver -Wl,--defsym=__gentoo_check_ldflags__=0 -ggdb3'
    libpth=/usr/local/lib64 /lib64 /usr/lib64 /usr/local/lib /usr/lib
    libs=-lgdbm -ldl -lm -lcrypt -lutil -lc -lgdbm_compat
    perllibs=-ldl -lm -lcrypt -lutil -lc
    libc=/lib/../lib64/libc.so.6
    so=so
    useshrplib=true
    libperl=libperl.so.5.42.0
    gnulibc_version='2.42'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E'
    cccdlflags='-fPIC'
    lddlflags='-shared -O3 -march=native -flto=jobserver -fno-semantic-interposition -mtls-dialect=gnu2 -g1 -pipe -fcf-protection=none -fdiagnostics-color=always -fdiagnostics-urls=never -frecord-gcc-switches -fdiagnostics-show-context=3 -Wa,-O2 -Wa,-mtune=znver2 -Warray-bounds --param=ggc-min-expand=120 -Wstrict-aliasing -Wfree-nonheap-object -Wuninitialized -Wmaybe-uninitialized -Wreturn-type -Wbuiltin-declaration-mismatch -Wtautological-compare -Werror=lto-type-mismatch -Werror=strict-aliasing -Werror=odr -Warray-bounds -Wstringop-overread -Wstringop-overflow -Wstringop-truncation -Wformat-overflow -Wformat-truncation -Wrestrict -ggdb3 -Wformat -Werror=format-security -fno-strict-aliasing -O3 -march=native -flto=jobserver -fno-semantic-interposition -mtls-dialect=gnu2 -g1 -pipe -fcf-protection=none -fdiagnostics-color=always -fdiagnostics-urls=never -frecord-gcc-switches -fdiagnostics-show-context=3 -Wa,-O2 -Wa,-mtune=znver2 -Warray-bounds --param=ggc-min-expand=120 -Werror=lto-type-mismatch -Werror=strict-aliasing -Werror=odr -Warray-bounds -Wstringop-overread -Wstringop-overflow -Wstringop-truncation -Wformat-overflow -Wformat-truncation -Wrestrict -Wl,-O1 -Wl,--as-needed -Wl,-z,pack-relative-relocs -flto=jobserver -Wl,--defsym=__gentoo_check_ldflags__=0 -ggdb3'


Characteristics of this binary (from libperl): 
  Compile-time options:
    HAS_LONG_DOUBLE
    HAS_STRTOLD
    HAS_TIMES
    NO_PERL_RAND_SEED
    PERLIO_LAYERS
    PERL_COPY_ON_WRITE
    PERL_DONT_CREATE_GVSV
    PERL_HASH_FUNC_SIPHASH13
    PERL_HASH_USE_SBOX32
    PERL_MALLOC_WRAP
    PERL_OP_PARENT
    PERL_PRESERVE_IVUV
    PERL_USE_SAFE_PUTENV
    USE_64_BIT_ALL
    USE_64_BIT_INT
    USE_LARGE_FILES
    USE_LOCALE
    USE_LOCALE_COLLATE
    USE_LOCALE_CTYPE
    USE_LOCALE_NUMERIC
    USE_LOCALE_TIME
    USE_PERLIO
    USE_PERL_ATOF
  Locally applied patches:
    0001-Fix-hpux-hints.patch
    - Fix hpux hints
    0002-aix-gcc-detection-and-shared-library-soname-support.patch
    - aix gcc detection and shared library soname support
    0003-cpan-ExtUtils-MakeMaker-drop-PORTAGE_TMPDIR-from-LD_.patch
    - cpan/ExtUtils-MakeMaker: drop $PORTAGE_TMPDIR from LD_RUN_PATH
    - Bug: https://bugs.gentoo.org/105054
    0004-Remove-rpath-and-append-LDFLAGS-to-lddlflags.patch
    - Remove -rpath and append LDFLAGS to lddlflags
    0005-Add-headers-for-opensolaris.patch
    - Add headers for opensolaris
    0006-List-packaged-patches-in-patchlevel.h.patch
    - List packaged patches for perl-5.42.0-r1(perl-5.42.0-patches-1) in patchlevel.h
    - Bug: https://bugs.debian.org/567489
    0007-Cleanup-PATH-and-shrpenv.patch
    - Cleanup PATH and shrpenv
    0008-Tweak-enc2xs-to-follow-symlinks-and-ignore-missing-I.patch
    - Tweak enc2xs to follow symlinks and ignore missing @INC directories.
    - Bug: https://bugs.debian.org/290336
    0009-darwin-Use-CC-to-link.patch
    - darwin: Use $CC to link
    - Bug: https://bugs.gentoo.org/297751
    0010-Provide-a-sensible-INSTALLDIRS-default-for-modules-i.patch
    - Provide a sensible INSTALLDIRS default for modules installed from CPAN.
    0011-Fix-interix-hints.patch
    - Fix interix hints
    0012-Set-libperl-soname.patch
    - Set libperl soname
    - Bug: https://bugs.gentoo.org/286840
    0013-Add-etc-perl-to-INC.patch
    - Add /etc/perl to @INC
    0014-cpan-ExtUtils-MakeMaker-remove-targets-that-generate.patch
    - cpan/ExtUtils-MakeMaker: remove targets that generate perllocal.pod
    0015-Don-t-force-fstack-protector-on-everyone.patch
    - Don't force -fstack-protector on everyone
    - Bug: https://bugs.gentoo.org/348557
    0016-Do-not-set-custom-CFLAGS-in-cpan-Digest-SHA.patch
    - Do not set custom CFLAGS in cpan/Digest-SHA
    - Bug: https://bugs.gentoo.org/506818
    0017-Configure-Don-t-include-sources-in-usr-local-for-com.patch
    - Configure: Don't include sources in /usr/local/ for compiling perl
    0018-cpan-IO-Socket-IP-Disable-network-tests.patch
    - cpan/IO-Socket-IP: Disable network tests
    0019-Fix-EUMM-podlocal-tests.patch
    - Fix EUMM podlocal tests
    0020-Skip-auto-linking-cl-and-pthread.patch
    - Skip auto-linking nsl and cl
    - Bug: https://rt.perl.org/Public/Bug/Display.html?id=13148
    0021-Disable-porting-tests-which-create-fun-false-failure.patch
    - Disable porting tests which create fun false-failures all over travis
    0022-Disable-PathTools-tests-which-fails-under-sandboxing.patch
    - Disable PathTools tests which fails under sandboxing
    - Bug: https://bugs.gentoo.org/645084
    0023-Fix-CPAN-FirstTime-defaults-with-nonexisting-site-di.patch
    - Fix CPAN::FirstTime defaults with nonexisting site dirs if a parent is writable
    - Bug: https://bugs.debian.org/688842
    0024-Pass-LD-settings-through-to-subdirectories.patch
    - Pass LD settings through to subdirectories
    - Bug: https://bugs.debian.org/758471
    0025-Respect-umask-during-installation.patch
    - Respect umask during installation
    0026-Fix-parallel-building.patch
    - Fix parallel building
    0027-Add-support-for-Dgentoolibdirs-note-hashes-in-uconfi.patch
    - Add support for -Dgentoolibdirs
    100-newFOROP-fix-crash-when-optimizing-2-var-for-over-bu.patch
    - Fix for keyword segfaulting when iterating over multiple values at a time
    - Bug: https://bugs.gentoo.org/964379
    - Bug: https://github.com/Perl/perl5/issues/23405
    101-class.c-gracefully-handle-reader-writer-after-strict.patch
    - Gracefully handle reader definition after strict error
    - Bug: https://github.com/Perl/perl5/issues/23511
    102-use-5.41-affects-current-line-source-encoding.patch
    - 5.41 use affects current line source::encoding
    - Bug: https://github.com/Perl/perl5/issues/23881
    103-Turn-off-POSIX-2008-locales-on-AIX.patch
    - Turn off POSIX 2008 locales on AIX
    - Bug: https://github.com/Perl/perl5/issues/23825
  Built under linux
  Compiled at Dec 13 2025 13:55:24
  @INC:
    /etc/perl
    /usr/local/lib64/perl5/5.42/x86_64-linux
    /usr/local/lib64/perl5/5.42
    /usr/lib64/perl5/vendor_perl/5.42/x86_64-linux
    /usr/lib64/perl5/vendor_perl/5.42
    /usr/lib64/perl5/5.42/x86_64-linux
    /usr/lib64/perl5/5.42
```

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Uninitialised memory use when Dumping undef'd string #24008

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Uninitialised memory use when Dumping undef'd string #24008

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