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

[Nikolay Edigaryev via Unbound-users]

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,