TLS: fix use-of-uninitialized-value error (#1566)

```
Uninitialized bytes in __interceptor_strlen at offset 0 inside [0x7ffc987443a0, 7)
==282918==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x5651277e001b in ndpi_strdup /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:268:13
    #1 0x5651277a45c1 in ndpi_set_risk /home/ivan/svnrepos/nDPI/src/lib/ndpi_utils.c:2254:12
    #2 0x5651278cfaa6 in processClientServerHello /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:1515:8
    #3 0x5651278e7834 in processTLSBlock /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:865:5
    #4 0x5651278e60bd in ndpi_search_tls_tcp /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:1024:2
    #5 0x5651278e1ffc in ndpi_search_tls_wrapper /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:2445:5
    #6 0x565127831c2a in check_ndpi_detection_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5150:6
    #7 0x565127832938 in check_ndpi_tcp_flow_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5198:12
    #8 0x5651278323a1 in ndpi_check_flow_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5217:12
    #9 0x565127843e3a in ndpi_detection_process_packet /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:6076:15
    #10 0x56512778641f in LLVMFuzzerTestOneInput /home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet.c:29:5
    #11 0x5651277874ae in main /home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet.c:101:17
    #12 0x7f4c46794082 in __libc_start_main /build/glibc-SzIz7B/glibc-2.31/csu/../csu/libc-start.c:308:16
    #13 0x5651276fe40d in _start (/home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet_with_main+0xa340d)
```
Found by oss-fuzzer
See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=47728

src/lib/protocols/tls.c

@@ -1488,6 +1488,7 @@ int processClientServerHello(struct ndpi_detection_module_struct *ndpi_struct,
 	  if(tot_alpn_len > packet->payload_packet_len)
 	    return 0;
 
+	  alpn_str[0] = '\0';
 	  while(s_offset < tot_alpn_len && s_offset < total_len) {
 	    u_int8_t alpn_i, alpn_len = packet->payload[s_offset++];