netfilter: ipset: Null pointer exception in ipset list:set

If we use before/after to add an element to an empty list it will cause
a kernel panic.

$> cat crash.restore
create a hash:ip
create b hash:ip
create test list:set timeout 5 size 4
add test b before a

$> ipset -R < crash.restore

Executing the above will crash the kernel.
Signed-off-by: Vishwanath Pai <vpai@akamai.com>
Reviewed-by: Josh Hunt <johunt@akamai.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>

net/netfilter/ipset/ip_set_list_set.c

@@ -260,11 +260,14 @@ list_set_uadd(struct ip_set *set, void *value, const struct ip_set_ext *ext,
 		else
 			prev = e;
 	}
-	/* Re-add already existing element */
-	if (n) {
+
+	/* If before/after is used on an empty set */
 	if ((d->before > 0 && !next) ||
 	    (d->before < 0 && !prev))
 		return -IPSET_ERR_REF_EXIST;
+
+	/* Re-add already existing element */
+	if (n) {
 		if (!flag_exist)
 			return -IPSET_ERR_EXIST;
 		/* Update extensions */