Fix dnstap relaying "random" messages instead of resolver/forwarder responses

[Nikolay Edigaryev]

This was already fixed for serviced_tcp_callback() in revision 3390, but
for some reason serviced_udp_callback() left untouched.

Index: services/outside_network.c
===================================================================
--- services/outside_network.c	(revision 3866)
+++ services/outside_network.c	(working copy)
@@ -1879,7 +1879,7 @@
 		return 0;
 	}
 #ifdef USE_DNSTAP
-	if(outnet->dtenv &&
+	if(error == NETEVENT_NOERROR && outnet->dtenv &&
 	   (outnet->dtenv->log_resolver_response_messages ||
 	    outnet->dtenv->log_forwarder_response_messages))
 		dt_msg_send_outside_response(outnet->dtenv, &sq->addr, c->type,