Avoid a PHP notice on Edit Menus screen when a menu is attached to a non-existing location.

props nofearinc.
fixes #26287.
Built from https://develop.svn.wordpress.org/trunk@26634


git-svn-id: http://core.svn.wordpress.org/trunk@26524 1a063a9b-81f0-0310-95a4-ce76da25c4cd