Don't return arcount=1 if EDNS0 RR won't fit in the packet.

Omitting the EDNS0 RR but setting arcount gives a malformed packet.
Also, don't accept UDP packet size less than 512 in recieved EDNS0.

src/forward.c

@@ -1412,6 +1412,8 @@ void receive_query(struct listener *listen, time_t now)
 	 defaults to 512 */
       if (udp_size > daemon->edns_pktsz)
 	udp_size = daemon->edns_pktsz;
+      else if (udp_size < PACKETSZ)
+	udp_size = PACKETSZ; /* Sanity check - can't reduce below default. RFC 6891 6.2.3 */
     }
 
 #ifdef HAVE_AUTH