If the decoder finishes decoding the current song between the two
IsIdle() checks, MPD stops playback instead of starting the decoder
for the next song.

This is usually not visible problem, because the main thread restarts
it via playlist::ResumePlayback(), but that way it, ignores "single"
mode.

As a workaround, this commit adds another "queued" check which
re-enters the player loop and checks again whether to start the
decoder.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/556