netfilter: nfnetlink_log: silence bogus compiler warning

net/netfilter/nfnetlink_log.c:800:18: warning: variable 'ctinfo' is uninitialized

The warning is bogus, the variable is only used if ct is non-NULL and
always initialised in that case.  Init to 0 too to silence this.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202309100514.ndBFebXN-lkp@intel.com/
Signed-off-by: Florian Westphal <fw@strlen.de>