ffmpeg seeking can cause playback exit

[danielk]

When ffmpeg finds a timestamp near the start of a file that is greater than the timestamp it finds near the end of a file during a seek it fails to perform the seek. But it doesn't _just_ fail to perform the seek it also leaves the file pointer at the end of the file so on our next read we encounter an EOF condition.

I'm attaching a patch that handles this failure mode with rollback to the original read position, but ideally ffmpeg would handle this seek.

[danielk]

Initial patch

[paulh]

Four year old ticket likely fixed by one of the ffmpeg syncs since then.