xen-netfront: Avoid deref'ing skbafter it is potentially freed.

xennet_tx_bug_gc can free the skb before we use it, so make sure we don't.

Signed-off-by: Keir Fraser <keir@xensource.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>