VLCPlaybackInfoSubtitlesFetcherViewController: Fix crash on iPad

UIAlertController with UIModalPresentationPopover as a style need to have sourceView/sourceRect set otherwise it will crash.

More info on the exception raised by the crash.

*** Terminating app due to uncaught exception 'NSGenericException', reason: 'Your application has presented a UIAlertController (<UIAlertController: 0x1461e1c00>) of style UIAlertControllerStyleActionSheet from UINavigationController (<UINavigationController: 0x146b54400>). The modalPresentationStyle of a UIAlertController with this style is UIModalPresentationPopover. You must provide location information for this popover through the alert controller's popoverPresentationController. You must provide either a sourceView and sourceRect or a barButtonItem. If this info