[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: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netfilter/nfnetlink_log.c

@@ -486,7 +486,7 @@ __build_packet_message(struct nfulnl_instance *inst,
 			 * for physical device (when called from ipv4) */
 			NFA_PUT(inst->skb, NFULA_IFINDEX_OUTDEV,
 				sizeof(tmp_uint), &tmp_uint);
-			if (skb->nf_bridge) {
+			if (skb->nf_bridge && skb->nf_bridge->physoutdev) {
 				tmp_uint =
 				    htonl(skb->nf_bridge->physoutdev->ifindex);
 				NFA_PUT(inst->skb, NFULA_IFINDEX_PHYSOUTDEV,