DNS: fix Index-out-of-bounds error (#2644)

```
Running: /home/ivan/Downloads/clusterfuzz-testcase-minimized-fuzz_ndpi_reader_pl7m_simplest_internal-5759495480868864
protocols/dns.c:482:5: runtime error: index 4 out of bounds for type 'u_int8_t[4]' (aka 'unsigned char[4]')
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior protocols/dns.c:482:5
protocols/dns.c:483:5: runtime error: index 4 out of bounds for type 'u_int32_t[4]' (aka 'unsigned int[4]')
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior protocols/dns.c:483:5
protocols/dns.c:490:12: runtime error: index 4 out of bounds for type 'u_int32_t[4]' (aka 'unsigned int[4]')

```
Found by oss-fuzz
See: https://issues.oss-fuzz.com/issues/383911300?pli=1

src/lib/protocols/dns.c

@@ -475,21 +475,26 @@ static int search_valid_dns(struct ndpi_detection_module_struct *ndpi_struct,
 			 || ((rsp_type == 0x1c) && (data_len == 16)) /* AAAA */
 			 )) {
 		if(found == 0) {
-		  /* Necessary for IP address comparison */
-		  memset(&flow->protos.dns.rsp_addr[flow->protos.dns.num_rsp_addr], 0, sizeof(ndpi_ip_addr_t));
 
-		  memcpy(&flow->protos.dns.rsp_addr[flow->protos.dns.num_rsp_addr], packet->payload + x, data_len);
-		  flow->protos.dns.is_rsp_addr_ipv6[flow->protos.dns.num_rsp_addr] = (data_len == 16) ? 1 : 0;
-		  flow->protos.dns.rsp_addr_ttl[flow->protos.dns.num_rsp_addr] = ttl;
-
-		  if(ndpi_struct->cfg.address_cache_size)
-		    ndpi_cache_address(ndpi_struct,
-				       flow->protos.dns.rsp_addr[flow->protos.dns.num_rsp_addr],
-				       flow->host_server_name,
-				       packet->current_time_ms/1000,
-				       flow->protos.dns.rsp_addr_ttl[flow->protos.dns.num_rsp_addr]);		
+		  if(flow->protos.dns.num_rsp_addr < MAX_NUM_DNS_RSP_ADDRESSES) {
+		    /* Necessary for IP address comparison */
+		    memset(&flow->protos.dns.rsp_addr[flow->protos.dns.num_rsp_addr], 0, sizeof(ndpi_ip_addr_t));
 
-		  if(++flow->protos.dns.num_rsp_addr == MAX_NUM_DNS_RSP_ADDRESSES)
+		    memcpy(&flow->protos.dns.rsp_addr[flow->protos.dns.num_rsp_addr], packet->payload + x, data_len);
+		    flow->protos.dns.is_rsp_addr_ipv6[flow->protos.dns.num_rsp_addr] = (data_len == 16) ? 1 : 0;
+		    flow->protos.dns.rsp_addr_ttl[flow->protos.dns.num_rsp_addr] = ttl;
+
+		    if(ndpi_struct->cfg.address_cache_size)
+		      ndpi_cache_address(ndpi_struct,
+				         flow->protos.dns.rsp_addr[flow->protos.dns.num_rsp_addr],
+				         flow->host_server_name,
+				         packet->current_time_ms/1000,
+				         flow->protos.dns.rsp_addr_ttl[flow->protos.dns.num_rsp_addr]);
+
+		    ++flow->protos.dns.num_rsp_addr;
+		  }
+
+		  if(flow->protos.dns.num_rsp_addr >= MAX_NUM_DNS_RSP_ADDRESSES)
 		    found = 1;
 		}
 	      }