Phantom cover art

[graue]

If an album had cover art during a previous DB update, but the image has been deleted or moved/renamed since, a new DB update keeps the stale path. It should clear the old filename and pick up the new one if there is one.

Repro:

1. Put an album under the music dir that has cover art
2. Update DB
3. Move album to different directory under music dir
4. Update DB
5. Play track from album

Expected result: Album art displays correctly

Actual result: Album art doesn't display because still pointing to old path