netfilter: nfnetlink_queue: silence bogus compiler warning

net/netfilter/nfnetlink_queue.c:601:36: warning: variable 'ctinfo' is
uninitialized when used here [-Wuninitialized]
   if (ct && nfnl_ct->build(skb, ct, ctinfo, NFQA_CT, NFQA_CT_INFO) < 0)

ctinfo is only uninitialized if ct == NULL.  Init it to 0 to silence this.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>