netfilter: nf_ct_expect: remove the redundant slash when policy name is empty

The 'name' filed in struct nf_conntrack_expect_policy{} is not a
pointer, so check it is NULL or not will always return true. Even if the
name is empty, slash will always be displayed like follows:
  # cat /proc/net/nf_conntrack_expect
  297 l3proto = 2 proto=6 src=1.1.1.1 dst=2.2.2.2 sport=1 dport=1025 ftp/
                                                                        ^

Fixes: 3a8fc53a ("netfilter: nf_ct_helper: allocate 16 bytes for the helper and policy names")
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_expect.c

@@ -574,7 +574,7 @@ static int exp_seq_show(struct seq_file *s, void *v)
 	helper = rcu_dereference(nfct_help(expect->master)->helper);
 	if (helper) {
 		seq_printf(s, "%s%s", expect->flags ? " " : "", helper->name);
-		if (helper->expect_policy[expect->class].name)
+		if (helper->expect_policy[expect->class].name[0])
 			seq_printf(s, "/%s",
 				   helper->expect_policy[expect->class].name);
 	}