netfilter: ctnetlink: avoid null pointer dereference

Dan Carpenter points out that deref occurs after NULL check, we should
re-fetch the pointer and check that instead.

Fixes: 2c205dd3

 ("netfilter: add struct nf_nat_hook and use it")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>