firewall: fix aliases crash report

This page has been reworked on master so this is a tiny cherry-pick
to bridge the gap to 15.7.1x when we merge it in.

src/www/firewall_aliases.php

@@ -31,10 +31,10 @@
 require_once("guiconfig.inc");
 require_once("filter.inc");
 
-if (!is_array($config['aliases'])) {
+if (!isset($config['aliases'])) {
         $config['aliases'] = array();
 }
-if (!is_array($config['aliases']['alias'])) {
+if (!isset($config['aliases']['alias'])) {
 	$config['aliases']['alias'] = array();
 }
 $a_aliases = &$config['aliases']['alias'];