src: fix `Illegal string offset' warning in interfaces

src/www/interfaces.php

@@ -228,7 +228,7 @@ $pconfig['dhcp_plus'] = isset($wancfg['dhcp_plus']);
 $pconfig['descr'] = remove_bad_chars($wancfg['descr']);
 $pconfig['enable'] = isset($wancfg['enable']);
 
-if (is_array($config['aliases']['alias'])) {
+if (isset($config['aliases']['alias'])) {
 	foreach($config['aliases']['alias'] as $alias) {
 		if($alias['name'] == $wancfg['descr']) {
 			$input_errors[] = sprintf(gettext("Sorry, an alias with the name %s already exists."),$wancfg['descr']);