Menus: When adding a label for special pages in `wp_setup_nav_menu_item()`,...

Menus: When adding a label for special pages in `wp_setup_nav_menu_item()`, check if `get_post_states()` is available, to avoid fatal errors on front end.

Fixes #49374.
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git-svn-id: https://core.svn.wordpress.org/trunk@47013 1a063a9b-81f0-0310-95a4-ce76da25c4cd