Do not crash when there is no current movie player controller

The app would crash when going to background very quickly after presenting a XCDYouTubeVideoPlayerViewController, i.e. `UIApplicationWillResignActiveNotification` is received before `MPMoviePlayerNowPlayingMovieDidChangeNotification`.
Interestingly, `objc_setAssociatedObject(nil, key, nil, …)` works fine but `objc_setAssociatedObject(nil, key, someNonNilObject, …)` crashes and this is exactly what happens when `currentMoviePlayerController` is still nil but we already have a `playerLayer` object.

Fixes #44