fuzz/sigpcap: initialize empty packet pool

Fixes runs with --enable-debug-validation. The target did not init a
packet pool, so for a tunnel packet would try to get a packet from
an uninitialized pool. In non-debug mode, this silently works by
falling back to a packet from alloc.

    (gdb) bt
    #0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
    #1  0x00007ffff35a6801 in __GI_abort () at abort.c:79
    #2  0x00007ffff359639a in __assert_fail_base (fmt=0x7ffff371d7d8 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=assertion@entry=0x555557fe7260 "!(pool->initialized == 0)",
        file=file@entry=0x555557fe7220 "tmqh-packetpool.c", line=line@entry=253, function=function@entry=0x555557fe7500 <__PRETTY_FUNCTION__.21181> "PacketPoolGetPacket") at assert.c:92
    #3  0x00007ffff3596412 in __GI___assert_fail (assertion=0x555557fe7260 "!(pool->initialized == 0)", file=0x555557fe7220 "tmqh-packetpool.c", line=253,
        function=0x555557fe7500 <__PRETTY_FUNCTION__.21181> "PacketPoolGetPacket") at assert.c:101
    #4  0x00005555577e24be in PacketPoolGetPacket () at tmqh-packetpool.c:253
    #5  0x0000555556914ecd in PacketGetFromQueueOrAlloc () at decode.c:183
    #6  0x00005555569161e1 in PacketTunnelPktSetup (tv=0x555559863980 <tv>, dtv=0x614000068e40, parent=0x61e0000fc080, pkt=0x61e0000fc470 "LL", len=72, proto=DECODE_TUNNEL_IPV4) at decode.c:286
    #7  0x00005555569de694 in DecodeIPv4inIPv6 (tv=0x555559863980 <tv>, dtv=0x614000068e40, p=0x61e0000fc080, pkt=0x61e0000fc470 "LL", plen=72) at decode-ipv6.c:59
    #8  0x00005555569e60b5 in DecodeIPV6ExtHdrs (tv=0x555559863980 <tv>, dtv=0x614000068e40, p=0x61e0000fc080, pkt=0x61e0000fc470 "LL", len=112) at decode-ipv6.c:522
    #9  0x00005555569e846f in DecodeIPV6 (tv=0x555559863980 <tv>, dtv=0x614000068e40, p=0x61e0000fc080, pkt=0x61e0000fc420 "cLL", len=255) at decode-ipv6.c:641
    #10 0x0000555556a032f9 in DecodeRaw (tv=0x555559863980 <tv>, dtv=0x614000068e40, p=0x61e0000fc080, pkt=0x61e0000fc420 "cLL", len=255) at decode-raw.c:70
    #11 0x0000555557659ba8 in DecodePcapFile (tv=0x555559863980 <tv>, p=0x61e0000fc080, data=0x614000068e40) at source-pcap-file.c:412
    #12 0x0000555556573401 in LLVMFuzzerTestOneInput (data=0x613000000047 "\241\262\315\064", size=339) at tests/fuzz/fuzz_sigpcap.c:158
    #13 0x0000555557a4dc66 in main (argc=2, argv=0x7fffffffdfa8) at tests/fuzz/onefile.c:51

That line:

    BUG_ON(pool->initialized == 0);

src/tests/fuzz/fuzz_sigpcap.c

@@ -92,6 +92,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
         tmm_modules[TMM_FLOWWORKER].ThreadInit(&tv, NULL, &fwd);
         StatsSetupPrivate(&tv);
 
+        PacketPoolInitEmpty();
         initialized = 1;
     }