Ensure that `wp.media.view.settings.post` is set before accessing the post's id

Avoids JS error in cases where media-editor is loaded individually
instead of with the whole `wp_enqueue_media()` process.

fixes #27836. props nacin

git-svn-id: https://develop.svn.wordpress.org/trunk@28149 602fd350-edb4-49c9-b593-d223f7449a82