Fix an heap-buffer-overflow

```
==32439==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x604000000080 at pc 0x564a9a1639ac bp 0x7ffc77d42e20 sp 0x7ffc77d42e18
READ of size 1 at 0x604000000080 thread T0
    #0 0x564a9a1639ab in ndpi_check_punycode_string /home/ivan/svnrepos/nDPI/src/lib/ndpi_utils.c:98:12
    ntop#1 0x564a9a26025a in ndpi_match_host_subprotocol /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:8652:6
    ntop#2 0x564a9a33e45c in ndpi_http_parse_subprotocol /home/ivan/svnrepos/nDPI/src/lib/protocols/http.c:446:9
    ntop#3 0x564a9a3354b8 in check_content_type_and_change_protocol /home/ivan/svnrepos/nDPI/src/lib/protocols/http.c:921:3
    ntop#4 0x564a9a32dc8b in process_request /home/ivan/svnrepos/nDPI/src/lib/protocols/http.c:1237:3
    ntop#5 0x564a9a329a9c in ndpi_check_http_tcp /home/ivan/svnrepos/nDPI/src/lib/protocols/http.c:1351:5
    ntop#6 0x564a9a3293ed in ndpi_search_http_tcp /home/ivan/svnrepos/nDPI/src/lib/protocols/http.c:1425:3
    ntop#7 0x564a9a21c64a in check_ndpi_detection_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5906:6
    ntop#8 0x564a9a21d057 in check_ndpi_tcp_flow_func /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:5954:12
```

Found by oss-fuzzer.
See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=59393

src/lib/ndpi_utils.c

@@ -91,13 +91,14 @@ _Static_assert(sizeof(struct ndpi_str_hash) == sizeof(struct ndpi_str_hash_priva
 int ndpi_check_punycode_string(char * buffer , int len) {
   int i = 0;
 
-  while(i++ < len - 3) {
+  while(i < len - 3) {
     if((buffer[i] == 'x')
        && (buffer[i+1] == 'n')
        && (buffer[i+2] == '-')
        && (buffer[i+3] == '-'))
       // is a punycode string
       return(1);
+    i++;
   }
 
   // not a punycode string