TLS: fix use-of-uninitialized-value error

Proper fix for the error already reported in 9040bc7

```
Uninitialized bytes in __interceptor_strlen at offset 3 inside [0x7ffc7a147390, 4)
==111876==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x55e3e4f32e5b in ndpi_strdup /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:268:13
    ntop#1 0x55e3e4ef7391 in ndpi_set_risk /home/ivan/svnrepos/nDPI/src/lib/ndpi_utils.c:2254:12
    ntop#2 0x55e3e5022fdf in processClientServerHello /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:1523:8
    ntop#3 0x55e3e503af44 in processTLSBlock /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:865:5
    ntop#4 0x55e3e50397cd in ndpi_search_tls_tcp /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:1024:2
    ntop#5 0x55e3e503570c in ndpi_search_tls_wrapper /home/ivan/svnrepos/nDPI/src/lib/protocols/tls.c:2453:5
    ntop#6 0x55e3e4f84a6a in check_ndpi_detection_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5150:6
    ntop#7 0x55e3e4f85778 in check_ndpi_tcp_flow_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5198:12
    ntop#8 0x55e3e4f851e1 in ndpi_check_flow_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5217:12
    ntop#9 0x55e3e4f96c7a in ndpi_detection_process_packet /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:6076:15
    ntop#10 0x55e3e4ed91ef in LLVMFuzzerTestOneInput /home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet.c:29:5
    ntop#11 0x55e3e4eda27e in main /home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet.c:101:17
    ntop#12 0x7f5cb3146082 in __libc_start_main /build/glibc-SzIz7B/glibc-2.31/csu/../csu/libc-start.c:308:16
    ntop#13 0x55e3e4e5140d in _start (/home/ivan/svnrepos/nDPI/fuzz/fuzz_process_packet_with_main+0xa340d) (BuildId: 0c02c433e039970dd13a60382b94dd5a8e19f625)
```

src/lib/protocols/tls.c

@@ -1495,7 +1495,6 @@ 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++];
 
@@ -1516,10 +1515,12 @@ int processClientServerHello(struct ndpi_detection_module_struct *ndpi_struct,
 
 	        s_offset += alpn_len, alpn_str_len += alpn_len;;
 	      } else {
+	        alpn_str[alpn_str_len] = '\0';
 	        ndpi_set_risk(ndpi_struct, flow, NDPI_TLS_UNCOMMON_ALPN, alpn_str);
 	        break;
 	      }
 	    } else {
+	      alpn_str[alpn_str_len] = '\0';
 	      ndpi_set_risk(ndpi_struct, flow, NDPI_TLS_UNCOMMON_ALPN, alpn_str);
 	      break;
 	    }