Spotted with the compiler flag -Wzero-as-null-pointer-constant.
Do not add this flag though, due to an occurrence in gperf generated
code:

    ../src/netlog/netlog-gperf.gperf: In function ‘netlog_gperf_lookup’:
    ../src/netlog/netlog-gperf.gperf:38:10: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]

Static asserts without a comment are a language extension.

The lookup table in hexchar() is of the size of its number of elements
and thus not implicitly NUL-terminated. Add the nonstring annotation to
please GCC 15:

    ../src/share/hexdecoct.c: In function ‘hexchar’:
    ../src/share/hexdecoct.c:38:39: warning: initializer-string for array of ‘char’ truncates NUL terminator but destination lacks ‘nonstring’ attribute (17 chars into 16 available) [-Wunterminated-string-initialization]
       38 |         static const char table[16] = "0123456789abcdef";
          |                                       ^~~~~~~~~~~~~~~~~~