A small but very critical bug.

src/net/udp.c

 		return -ENOMEM;
 	}
 	pkb_reserve ( conn->tx_pkb, UDP_MAX_HLEN );
-	conn->udp_op->senddata ( conn, conn->tx_pkb, 
+	conn->udp_op->senddata ( conn, conn->tx_pkb->data, 
 				 pkb_available ( conn->tx_pkb ) );
 	return 0;
 }