netfilter: nf_nat: fix crash in nf_nat_csum

Fix crash in nf_nat_csum when mangling packets
in OUTPUT hook where skb->dev is not defined, it is set
later before POSTROUTING. Problem happens for CHECKSUM_NONE.
We can check device from rt but using CHECKSUM_PARTIAL
should be safe (skb_checksum_help).
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/nf_nat_helper.c

@@ -160,7 +160,7 @@ static void nf_nat_csum(struct sk_buff *skb, const struct iphdr *iph, void *data
 
 	if (skb->ip_summed != CHECKSUM_PARTIAL) {
 		if (!(rt->rt_flags & RTCF_LOCAL) &&
-		    skb->dev->features & NETIF_F_V4_CSUM) {
+		    (!skb->dev || skb->dev->features & NETIF_F_V4_CSUM)) {
 			skb->ip_summed = CHECKSUM_PARTIAL;
 			skb->csum_start = skb_headroom(skb) +
 					  skb_network_offset(skb) +