netfilter: ctnetlink: fix uninitialized variable

net/netfilter/nf_conntrack_netlink.c: In function 'ctnetlink_nfqueue_attach_expect':
'helper' may be used uninitialized in this function

It was only initialized in if CTA_EXPECT_HELP_NAME attribute was
present, it must be NULL otherwise.

Problem added recently in bd077937
(netfilter: nfnetlink_queue: allow to attach expectations to conntracks).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>