[PATCH] net s2io: return on NULL dev_alloc_skb()

Checks for NULL dev_alloc_skb() and returns on true to avoid subsequent
dereference.

Cc: Jeff Garzik <jgarzik@pobox.com>
Cc: Christoph Hellwig <hch@infrared.org>
Signed-off-by: David Rientjes <rientjes@cs.washington.edu>
Signed-off-by: Jeff Garzik <jeff@garzik.org>