DNS: fix extraction of transactionID field

We can't write to `flow->protos.dns` until we are sure this is a valid
DNS packet

```
AddressSanitizer:DEADLYSIGNAL
=================================================================
==14729==ERROR: AddressSanitizer: SEGV on unknown address (pc 0x60e876372a86 bp 0x000000000000 sp 0x79392fdf90e0 T1)
==14729==The signal is caused by a READ memory access.
==14729==Hint: this fault was caused by a dereference of a high value address (see register values below).  Disassemble the provided pc to learn which register was used.
    #0 0x60e876372a86 in __asan::Allocator::Deallocate(void*, unsigned long, unsigned long, __sanitizer::BufferedStackTrace*, __asan::AllocType) (/home/ivan/svnrepos/nDPI/example/ndpiReader+0x8b0a86) (BuildId: a9c4718bcd5c3947812b6fd704e203b8bb6f633c)
    ntop#1 0x60e87640b29f in free (/home/ivan/svnrepos/nDPI/example/ndpiReader+0x94929f) (BuildId: a9c4718bcd5c3947812b6fd704e203b8bb6f633c)
    ntop#2 0x60e87647b0ec in free_wrapper /home/ivan/svnrepos/nDPI/example/ndpiReader.c:348:3
    ntop#3 0x60e876865454 in ndpi_free /home/ivan/svnrepos/nDPI/src/lib/ndpi_memory.c:82:7
    ntop#4 0x60e8767f0d4f in ndpi_free_flow_data /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:6752:2
    ntop#5 0x60e8767abd67 in ndpi_free_flow /home/ivan/svnrepos/nDPI/src/lib/ndpi_main.c:10449:5

```
Found by oss-fuzz

src/lib/protocols/dns.c

@@ -283,7 +283,7 @@ static int search_valid_dns(struct ndpi_detection_module_struct *ndpi_struct,
   memcpy(dns_header, (struct ndpi_dns_packet_header*)&packet->payload[x],
 	 sizeof(struct ndpi_dns_packet_header));
 
-  flow->protos.dns.transaction_id = dns_header->tr_id = ntohs(dns_header->tr_id);
+  dns_header->tr_id = ntohs(dns_header->tr_id);
   dns_header->flags = ntohs(dns_header->flags);
   dns_header->num_queries = ntohs(dns_header->num_queries);
   dns_header->num_answers = ntohs(dns_header->num_answers);
@@ -320,6 +320,7 @@ static int search_valid_dns(struct ndpi_detection_module_struct *ndpi_struct,
 	} else
 	  x++;
       }
+      flow->protos.dns.transaction_id = dns_header->tr_id;
     } else {
       if(flow->detected_protocol_stack[0] != NDPI_PROTOCOL_UNKNOWN)
         ndpi_set_risk(ndpi_struct, flow, NDPI_MALFORMED_PACKET, "Invalid DNS Header");
@@ -345,6 +346,7 @@ static int search_valid_dns(struct ndpi_detection_module_struct *ndpi_struct,
       }
     }
 
+    flow->protos.dns.transaction_id = dns_header->tr_id;
     flow->protos.dns.reply_code = dns_header->flags & 0x0F;
 
     if(flow->protos.dns.reply_code != 0) {