netfilter: ctnetlink: fix crash during expectation creation

This patch fixes a possible crash due to the missing initialization
of the expectation class when nf_ct_expect_related() is called.

Reported-by: BORBELY Zoltan <bozo@andrews.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>