audio: simplify condition

The expression added with the previous commit (0cce8fe6) looked slightly
more complicated than it has to be. The code is equivalent.
video: fix attached picture mode

Playing audio files with embedded cover art broke due to some of the
recent changes. Treat video EOF properly, and don't burn the CPU.
Disable hrseek for video in attached picture mode, since the decoder
will always produce a new image, which makes hrseek never terminate.

Fixes #970.