netfilter: nf_tables: fix chain filter in nf_tables_dump_rules()

ctx->chain may be null now that we have very large object names,
so we cannot check for ctx->chain[0] here.

Fixes: b7263e07 ("netfilter: nf_tables: Allow table names of up to 255 chars")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Phil Sutter <phil@nwl.cc>

net/netfilter/nf_tables_api.c

@@ -2072,7 +2072,7 @@ static int nf_tables_dump_rules(struct sk_buff *skb,
 				continue;
 
 			list_for_each_entry_rcu(chain, &table->chains, list) {
-				if (ctx && ctx->chain[0] &&
+				if (ctx && ctx->chain &&
 				    strcmp(ctx->chain, chain->name) != 0)
 					continue;