netlink_delinearize: Avoid potential null pointer deref

netlink_delinearize: Avoid potential null pointer deref

Phil Sutter says:

As netlink_get_register() may return NULL, we must not pass the returned
data unchecked to expr_set_type() as that will dereference it. Since the
parser has failed at that point anyway, by returning early we can skip
the useless statement allocation that follows in
netlink_parse_ct_stmt().

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Phil Sutter <phil@nwl.cc>