[NETFILTER]: nf_nat: fix MASQUERADE crash on device down

Check the return value of nfct_nat() in device_cmp(), we might very well
have non NAT conntrack entries as well (Netfilter bugzilla #528).

Signed-off-by: Martin Josefsson <[e-mail]>
Signed-off-by: Patrick McHardy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/netfilter/ipt_MASQUERADE.c

···
 static inline int
 device_cmp(struct ip_conntrack *i, void *ifindex)
 {
+	int ret;
 #ifdef CONFIG_NF_NAT_NEEDED
 	struct nf_conn_nat *nat = nfct_nat(i);
+
+	if (!nat)
+		return 0;
 #endif
-	int ret;
 
 	read_lock_bh(&masq_lock);
 #ifdef CONFIG_NF_NAT_NEEDED