netfilter: conntrack: fix NULL pointer dereference in nf_confirm_cthelper

An nf_conntrack_helper from nf_conn_help may become NULL after DNAT.

Observed when TCP port 1720 (Q931_PORT), associated with h323 conntrack
helper, is DNAT'ed to another destination port (e.g. 1730), while
nfqueue is being used for final acceptance (e.g. snort).

This happenned after transition from kernel 4.14 to 5.10.161.

Workarounds:
 * keep the same port (1720) in DNAT
 * disable nfqueue
 * disable/unload h323 NAT helper

$ linux-5.10/scripts/decode_stacktrace.sh vmlinux < /tmp/kernel.log
BUG: kernel NULL pointer dereference, address: 0000000000000084
[..]
RIP: 0010:nf_conntrack_update (net/netfilter/nf_conntrack_core.c:2080 net/netfilter/nf_conntrack_core.c:2134) nf_conntrack
[..]
nfqnl_reinject (net/netfilter/nfnetlink_queue.c:237) nfnetlink_queue
nfqnl_recv_verdict (net/netfilter/nfnetlink_queue.c:1230) nfnetlink_queue
nfnetlink_rcv_msg (net/netfilter/nfnetlink.c:241) nfnetlink
[..]

Fixes: ee04805f

 ("netfilter: conntrack: make conntrack userspace helpers work again")
Signed-off-by: Tijs Van Buggenhout <tijs.van.buggenhout@axsguard.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>