ffmpeg.ExtractImage returns a pipe-based reader immediately, before ffmpeg
finishes processing. When the audio file has no embedded image stream (e.g.
a plain MP3), ffmpeg exits with an error that closes the pipe asynchronously.
The selectImageReader function saw the non-nil reader as a success and
returned it instead of falling through to the next source in the chain
(album art). This caused getCoverArt to return an error response for tracks
on albums where the disc artwork reader was invoked but no embedded art
existed.
Fixed by reading one byte from the pipe to validate the stream delivers
data before returning it. If the read fails, the reader is closed and nil
is returned, allowing the fallback chain to continue to album artwork.
Closes#5265
Deluan
420d2c8e5a