Menu Customizer: custom menu items must not have a db id set (even of 0) to make it through `wp_save_nav_menu_items()`; unset it before calling that function.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@935466 b8457f37-d9ea-0310-8a92-e5e31aec5664