Put back code accidentaly removed in prev commit.

(cherry picked from commit 96aca84)

udp_server.c

@@ -374,6 +374,11 @@ int udp_rcv_loop(void)
 			else goto error;
 		}
 
+		if (len<MIN_UDP_PACKET) {
+			LM_DBG("probing packet received len = %d\n", len);
+			continue;
+		}
+
 		/* we must 0-term the messages, receive_msg expects it */
 		buf[len]=0; /* no need to save the previous char */