netfilter: nfnetlink_hook: fix array index out-of-bounds error

Currently the array net->nf.hooks_ipv6 is accessed by index hook
before hook is sanity checked. Fix this by moving the sanity check
to before the array access.

Addresses-Coverity: ("Out-of-bounds access")
Fixes: e2cf17d3 ("netfilter: add new hook nfnl subsystem")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>