Fix an obscure bug where moving a submenu item from a plugin menu that doesn't have a hook callback (i.e. an unusable menu serving as a placeholder) to a different menu would corrupte the menu item URL.
 
 Props to WooCommerce for exposing this bug through their highly unusual admin menu setup.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@996111 b8457f37-d9ea-0310-8a92-e5e31aec5664