remove_menu_page: Check for existing menu items

[sergej.mueller]

This error appears in my error log:

PHP Warning: Invalid argument supplied for foreach() in /www/htdocs/wp-admin/includes/plugin.php on line 1290

The if-empty patch is attached.

Looks like there are only two ways to get this warning:

1. Call remove_menu_page() after the admin is fully loaded, but before $menu global is set up, e.g. by hooking into auth_redirect action instead of admin_menu, which goes against the Codex example: ​http://codex.wordpress.org/Function_Reference/remove_menu_page.
2. Manually unset the $menu global at some point.

Both are developer errors, so the developer should see the warning.

Any foreach command should be checked "if empty".

Similar enhancements were suggested in #17299 and #18927.

The consensus was that we shouldn't hide warnings generated by plugin or theme bugs. That would only make debugging harder.