Previous versions of MPD would call SetVolume on enabled outputs before
they are ready, causing all of MPD to crash. Checking the really_enabled
flag prevents this, though it also prevents setting volume before the
player starts.

Before (with the PipeWire output):
  [i] ~$ mpc clear
  volume: 81%   repeat: off   random: off   single: off   consume: off
  [i] ~$ systemctl --user restart mpd.service
  [i] ~$ mpc volume 100
  MPD error: Connection closed by the server
  [i] ~ 1 $

After:
  [i] ~$ # mpd is freshly started w/o anything in the queue
  [i] ~$ mpc
  volume:100%   repeat: off   random: off   single: off   consume: off
  [i] ~$ mpc volume 80
  MPD error: problems setting volume
  [i] ~ 1 $ mpc
  volume:100%   repeat: off   random: off   single: off   consume: off
  [i] ~$