VLCMovieViewController: External: Check superview before constraint

Fixes https://appcenter.ms/orgs/videolan/apps/VLC-for-iOS-production/crashes/errors/3165170170u/overview.

WIP since this wasn't tested yet but it should do the trick.

We should make sure that it is the correct superview before setting constraint else it could crash.

For example, Chromecast.