netfilter: nf_nat: missing condition in nf_xfrm_me_harder()

This if statement was accidentally dropped in (aaa795ad netfilter:
nat: propagate errors from xfrm_me_harder()) so now it returns
unconditionally.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_nat_core.c

@@ -90,6 +90,7 @@ int nf_xfrm_me_harder(struct sk_buff *skb, unsigned int family)
 	int err;
 
 	err = xfrm_decode_session(skb, &fl, family);
+	if (err < 0)
 		return err;
 
 	dst = skb_dst(skb);