netfilter: nf_conntrack_helper: fix possible null deref during error log

netfilter: nf_conntrack_helper: fix possible null deref during error log

Reported by sashiko: there is a small race window.

If a helper module is unloaded or a userspace-defined helper is
removed, nf_conntrack_helper_unregister() sets ->helper to NULL.

Handle this safely.  This needs a second patch to close related
race during nf_conntrack_helper_unregister().

Fixes: b20ab9cc63ca ("netfilter: nf_ct_helper: better logging for dropped packets")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>