Fix invalid memory access

The bug is triggered when `pe_offset == (u_int32_t)-1`

```
AddressSanitizer:DEADLYSIGNAL
=================================================================
==23719==ERROR: AddressSanitizer: SEGV on unknown address 0x5081000002b3 (pc 0x55c69274ac72 bp 0x7ffffffc8e70 sp 0x7ffffffc8cc0 T0)
==23719==The signal is caused by a READ memory access.
    #0 0x55c69274ac72 in ndpi_search_portable_executable /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:8191:7
    ntop#1 0x55c69271606b in ndpi_internal_detection_process_packet /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:8596:5
    ntop#2 0x55c69270f58f in ndpi_detection_process_packet /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:8629:22
    ntop#3 0x55c6926a07e7 in LLVMFuzzerTestOneInput /home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet.c:24:5
    ntop#4 0x55c6925a79b6 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet+0x64e9b6) (BuildId: ec46c60ec7e03ebfb3d825bd6308d0a8d6e9803b)
    ntop#5 0x55c692590d48 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) (/home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet+0x637d48) (BuildId: ec46c60ec7e03ebfb3d825bd6308d0a8d6e9803b)
    ntop#6 0x55c69259685a in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet+0x63d85a) (BuildId: ec46c60ec7e03ebfb3d825bd6308d0a8d6e9803b)
    ntop#7 0x55c6925c0e02 in main (/home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet+0x667e02) (BuildId: ec46c60ec7e03ebfb3d825bd6308d0a8d6e9803b)
    ntop#8 0x7f8e99793082 in __libc_start_main /build/glibc-wuryBv/glibc-2.31/csu/../csu/libc-start.c:308:16
    ntop#9 0x55c69258baed in _start (/home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet+0x632aed) (BuildId: ec46c60ec7e03ebfb3d825bd6308d0a8d6e9803b)
```

Found by oss-fuzzer
See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=67881

src/lib/ndpi_main.c

@@ -8186,7 +8186,7 @@ static void ndpi_search_portable_executable(struct ndpi_detection_module_struct
   }
 
   uint32_t const pe_offset = le32toh(get_u_int32_t(packet->payload, 0x3C));
-  if (packet->payload_packet_len <= pe_offset + 4 ||
+  if ((u_int32_t)(packet->payload_packet_len - 4) <= pe_offset ||
       be32toh(get_u_int32_t(packet->payload, pe_offset)) != pe_signature)
   {
     return;