Fix a memory leak
```
==97697==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x55a6967cfa7e in malloc (/home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader+0x701a7e) (BuildId: c7124999fa1ccc54346fa7bd536d8eab88c3ea01)
    ntop#1 0x55a696972ab5 in ndpi_malloc /home/ivan/svnrepos/nDPI/src/lib/ndpi_memory.c:60:25
    ntop#2 0x55a696972da0 in ndpi_strdup /home/ivan/svnrepos/nDPI/src/lib/ndpi_memory.c:113:13
    ntop#3 0x55a696b7658d in processClientServerHello /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:2394:46
    ntop#4 0x55a696b86e81 in processTLSBlock /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:897:5
    ntop#5 0x55a696b80649 in ndpi_search_tls_udp /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:1262:11
    ntop#6 0x55a696b67a57 in ndpi_search_tls_wrapper /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:2751:5
    ntop#7 0x55a696b67758 in switch_to_tls /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:1408:3
    ntop#8 0x55a696c47810 in stun_search_again /home/ivan/svnrepos/nDPI/src/lib/protocols/stun.c:422:4
    ntop#9 0x55a6968a22af in ndpi_process_extra_packet /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:7247:9
    ntop#10 0x55a6968acd6f in ndpi_internal_detection_process_packet /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:7746:5
    ntop#11 0x55a6968aba3f in ndpi_detection_process_packet /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:8013:22
    ntop#12 0x55a69683d30e in packet_processing /home/ivan/svnrepos/nDPI/fuzz/../example/reader_util.c:1723:31
    ntop#13 0x55a69683d30e in ndpi_workflow_process_packet /home/ivan/svnrepos/nDPI/fuzz/../example/reader_util.c:2440:10
    ntop#14 0x55a69680f08f in LLVMFuzzerTestOneInput /home/ivan/svnrepos/nDPI/fuzz/fuzz_ndpi_reader.c:135:7
[...]
SUMMARY: AddressSanitizer: 16 byte(s) leaked in 1 allocation(s).
```
Found by oss-fuzzer
See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=64564