TLS: fix parsing of certificate elements (#1910)

```
==1228==ERROR: AddressSanitizer: SEGV on unknown address 0x6040000bed05 (pc 0x00000056e148 bp 0x7ffcca534320 sp 0x7ffcca5330c0 T0)
==1228==The signal is caused by a WRITE memory access.
    #0 0x56e148 in processCertificateElements ndpi/src/lib/protocols/tls.c:682:79
    #1 0x56c60f in LLVMFuzzerTestOneInput ndpi/fuzz/fuzz_tls_certificate.c:43:3
    #2 0x43de63 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
    #3 0x4295c2 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:324:6
    #4 0x42ee6c in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:860:9
    #5 0x4583a2 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    #6 0x7f8c021c9082 in __libc_start_main /build/glibc-SzIz7B/glibc-2.31/csu/libc-start.c:308:16
    #7 0x41f78d in _start
```

Found by oss-fuzz.
See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=57317

src/lib/protocols/tls.c

@@ -364,7 +364,8 @@ void processCertificateElements(struct ndpi_detection_module_struct *ndpi_struct
 				struct ndpi_flow_struct *flow,
 				u_int16_t p_offset, u_int16_t certificate_len) {
   struct ndpi_packet_struct *packet = &ndpi_struct->packet;
-  u_int16_t num_found = 0, i;
+  u_int16_t num_found = 0;
+  int32_t i;
   char buffer[64] = { '\0' }, rdnSeqBuf[2048];
   u_int rdn_len = 0;