If the next descriptor array entry cannot be allocated by dev_alloc_skb(),
return immediately so it is not dereferenced later.  We cannot register the
device with a partial descriptor list.

Cc: Mikael Starvik <starvik@axis.com>
Signed-off-by: David Rientjes <rientjes@cs.washington.edu>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>