player: don't clear command before do_play() returns

This bug caused the audio output devices to stay open, although MPD
wasn't playing: quitDecode() resetted player_control.command, assuming
that the command was STOP.  This way, player_task() didn't see the
CLOSE_AUDIO command, and the device was kept open.

Don't clear player_control.command in quitDecode().

src/player_thread.c

@@ -79,7 +79,6 @@ static void quitDecode(void)
 {
 	dc_stop(&pc.notify);
 	pc.state = PLAYER_STATE_STOP;
-	pc.command = PLAYER_COMMAND_NONE;
 	wakeup_main_task();
 }