netfilter: nf_tables: Fix dereference of null pointer flow

In the case where chain->flags & NFT_CHAIN_HW_OFFLOAD is false then
nft_flow_rule_create is not called and flow is NULL. The subsequent
error handling execution via label err_destroy_flow_rule will lead
to a null pointer dereference on flow when calling nft_flow_rule_destroy.
Since the error path to err_destroy_flow_rule has to cater for null
and non-null flows, only call nft_flow_rule_destroy if flow is non-null
to fix this issue.

Addresses-Coverity: ("Explicity null dereference")
Fixes: 3c5e4462 ("netfilter: nf_tables: memleak in hw offload abort path")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -3446,7 +3446,8 @@ static int nf_tables_newrule(struct sk_buff *skb, const struct nfnl_info *info,
 	return 0;
 
 err_destroy_flow_rule:
-	nft_flow_rule_destroy(flow);
+	if (flow)
+		nft_flow_rule_destroy(flow);
 err_release_rule:
 	nf_tables_rule_release(&ctx, rule);
 err_release_expr: