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.

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);