Properly check if an array key exists.

modules/menu/menu.module

@@ -756,7 +756,7 @@ function menu_parent_options($mid, $pid = 0, $depth = 0) {
   }
 
   // Add children of $pid to the list recursively.
-  if ($parent_item['children']) {
+  if (isset($parent_item['children'])) {
     usort($parent_item['children'], '_menu_sort');
     foreach ($parent_item['children'] as $child) {
       $options += menu_parent_options($mid, $child, $depth);