netfilter: nft_ct: fix null pointer in ct expectations support

nf_ct_helper_ext_add may return null, which must then be checked.

Fixes: 857b4602 ("netfilter: nft_ct: add ct expectations support")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Stéphane Veyret <sveyret@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>