[netdevice] Improve detection of bugs in drivers' TX completion handling

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/netdevice.c

 	}
 
 	/* Catch data corruption as early as possible */
-	assert ( iobuf->list.next != NULL );
-	assert ( iobuf->list.prev != NULL );
+	list_check_contains ( iobuf, &netdev->tx_queue, list );
 
 	/* Dequeue and free I/O buffer */
 	list_del ( &iobuf->list );