Remove proto == NULL checking since ip_conntrack_[nat_]proto_find_get
always returns a valid pointer. Fix missing ip_conntrack_proto_put in
some paths as well.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

-- 
The dawn of the fourth age of Linux firewalling is coming; a time of
great struggle and heroic deeds -- J.Kadlecsik got inspired by J.Morris