[NETFILTER]: nfnetlink_log: fix crash on bridged packet

physoutdev is only set on purely bridged packet, when nfnetlink_log is used
in the OUTPUT/FORWARD/POSTROUTING hooks on packets forwarded from or to a
bridge it crashes when trying to dereference skb->nf_bridge->physoutdev.

Reported by Holger Eitzenberger <heitzenberger@astaro.com>

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>