Don't fallback to default post type or taxonomy if given an invalid post type...

Don't fallback to default post type or taxonomy if given an invalid post type or taxonomy.  Use typenow as the canonical post type. Props nacin. see #19131

git-svn-id: https://core.svn.wordpress.org/trunk@19321 1a063a9b-81f0-0310-95a4-ce76da25c4cd