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>

net/netfilter/nfnetlink_hook.c

@@ -181,9 +181,9 @@ nfnl_hook_entries_head(u8 pf, unsigned int hook, struct net *net, const char *de
 		hook_head = rcu_dereference(net->nf.hooks_ipv4[hook]);
 		break;
 	case NFPROTO_IPV6:
-		hook_head = rcu_dereference(net->nf.hooks_ipv6[hook]);
 		if (hook >= ARRAY_SIZE(net->nf.hooks_ipv6))
 			return ERR_PTR(-EINVAL);
+		hook_head = rcu_dereference(net->nf.hooks_ipv6[hook]);
 		break;
 	case NFPROTO_ARP:
 #ifdef CONFIG_NETFILTER_FAMILY_ARP