mixer: explicitly close all mixers on shutdown

Mixers with the "global" flag set aren't closed automatically when the
output device is closed.  Thus, they might still be open when MPD
shuts down.

NEWS

@@ -3,6 +3,7 @@ ver 0.15.7 (2009/??/??)
   - id3: fix ID3v1 charset conversion
 * decoders:
   - ffmpeg: don't try to force stereo
+* mixer: explicitly close all mixers on shutdown
 * mapper: fix memory leak when playlist_directory is not set
 * mapper: apply filesystem_charset to playlists
 * command: verify playlist name in the "rm" command

src/mixer_control.c

@@ -62,6 +62,10 @@ mixer_free(struct mixer *mixer)
 	assert(mixer->plugin != NULL);
 	assert(mixer->mutex != NULL);
 
+	/* mixers with the "global" flag set might still be open at
+	   this point (see mixer_auto_close()) */
+	mixer_close(mixer);
+
 	g_mutex_free(mixer->mutex);
 
 	mixer->plugin->finish(mixer);