test/NullMixerListener: new class to fix -Wnull-dereference

test/NullMixerListener.hxx

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2003-2018 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef NULL_MIXER_LISTENER_HXX
+#define NULL_MIXER_LISTENER_HXX
+
+#include "mixer/Listener.hxx"
+
+class NullMixerListener : public MixerListener {
+public:
+	void OnMixerVolumeChanged(Mixer &, int) override {}
+};
+
+#endif