Custom rules: add missing check

Fix an use-of-uninitialized-value error

```
==28646==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x55a9b7a3dea2 in ndpi_patricia_lookup /home/ivan/svnrepos/nDPI/src/lib/third_party/src/ndpi_patricia.c:739:8
    ntop#1 0x55a9b7442cac in add_to_ptree /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:2588:10
    ntop#2 0x55a9b7469290 in ndpi_add_ip_risk_mask /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:4161:12
    ntop#3 0x55a9b746cf14 in ndpi_handle_rule /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:4297:11
    ntop#4 0x55a9b74842ae in ndpi_load_protocols_file_fd /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:4944:8
    ntop#5 0x55a9b7424706 in LLVMFuzzerTestOneInput /home/ivan/svnrepos/nDPI/fuzz/fuzz_filecfg_protocols.c:38:3
[...]
```

Found by oss-fuzz
See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=63754

src/lib/ndpi_main.c

@@ -4151,13 +4151,15 @@ int ndpi_add_ip_risk_mask(struct ndpi_detection_module_struct *ndpi_str,
   if(!is_ipv6 && ndpi_str->ip_risk_mask_ptree) {
     struct in_addr pin;
 
-    pin.s_addr = inet_addr(addr);
+    if(inet_pton(AF_INET, addr, &pin) != 1)
+      return(-1);
     node = add_to_ptree(ndpi_str->ip_risk_mask_ptree, AF_INET,
 			&pin, cidr ? atoi(cidr) : 32 /* bits */);
   } else if(is_ipv6 && ndpi_str->ip_risk_mask_ptree6) {
     struct in6_addr pin6;
 
-    inet_pton(AF_INET6, addr, &pin6);
+    if(inet_pton(AF_INET6, addr, &pin6) != 1)
+      return(-1);
     node = add_to_ptree(ndpi_str->ip_risk_mask_ptree6, AF_INET6,
 			&pin6, cidr ? atoi(cidr) : 128 /* bits */);
   } else {