Ogg file with Theora cover issue

[clsid2]

There are Ogg files in the wild that use a single frame Theora video stream instead of proper coverart.

Sample file

First issue is a crash with multi-threaded decoding. Fix here. There already is issue for it on FFmpeg bug tracker.

Second issue is that seeking does not work properly. It just starts from beginning upon each seek, even though it does jump in timeline.

File plays without issue in Firefox.