netfilter: nf_tables: don't update chain with unset counters

commit b88825de upstream.

Fix possible replacement of the per-cpu chain counters by null
pointer when updating an existing chain in the commit path.

Reported-by: Matteo Croce <technoboy85@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>