[PATCH] fix possible NULL ptr deref in forcedeth

There seems to be a possible NULL pointer deref bug in
drivers/net/forcedeth.c::nv_loopback_test().  If dev_alloc_skb() fails, the
next line will call skb_put() with a NULL first argument which it'll then
try to deref - kaboom: a NULL pointer deref.  Found by coverity (#1337).

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Cc: Ayaz Abdulla <aabdulla@nvidia.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Stephen Hemminger <shemminger@osdl.org>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>