netfilter: ctnetlink: fix leak in error path of ctnetlink_create_expect

This patch fixes a leak in one of the error paths of
ctnetlink_create_expect if no helper and no timeout is specified.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>