sunvnet: fix potential NULL pointer dereference

One of the error cases for vnet_start_xmit()'s "out_dropped" label
is port == NULL, so only mess with port->clean_timer when port is not NULL.

Signed-off-by: David L Stevens <david.stevens@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sun/sunvnet.c

@@ -1073,7 +1073,7 @@ out_dropped:
 	if (pending)
 		(void)mod_timer(&port->clean_timer,
 				jiffies + VNET_CLEAN_TIMEOUT);
-	else
+	else if (port)
 		del_timer(&port->clean_timer);
 	dev->stats.tx_dropped++;
 	return NETDEV_TX_OK;