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

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>

net/netfilter/nf_tables_api.c

@@ -899,6 +899,9 @@ static struct nft_stats __percpu *nft_stats_alloc(const struct nlattr *attr)
 static void nft_chain_stats_replace(struct nft_base_chain *chain,
 				    struct nft_stats __percpu *newstats)
 {
+	if (newstats == NULL)
+		return;
+
 	if (chain->stats) {
 		struct nft_stats __percpu *oldstats =
 				nft_dereference(chain->stats);