Fix crash when trying to use a video stream of a placeholder
Fix crash when trying to use a video stream of a placeholder

If the video is supposed to be loaded via netstream, a placeholder object is
created. Don't play if this is detected. Also don't play if the video uses an
unsupported codec.

freedesktop-unofficial-mirror/swfdec__swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,mltframework/swfdec,mltframework/swfdec