leave it in that state.  Likewise, if an audio output is in state
DEVICE_ON, and reopening the device due to a format change fails, change it
to state DEVICE_ENABLE.  This will prevent flushAudioBuffer from even
attempting to play audio on a closed device (even though it would fail
anyway).

git-svn-id: https://svn.musicpd.org/mpd/trunk@6529 09075e82-0dd4-0310-85a5-a0d7c8717e4f