pcm_volume: return bool

Don't abort MPD when a sample format is not supported by pcm_volume().

test/run_decoder.c

@@ -30,11 +30,12 @@
 /**
  * No-op dummy.
  */
-void
+bool
 pcm_volume(G_GNUC_UNUSED void *buffer, G_GNUC_UNUSED int length,
 	   G_GNUC_UNUSED const struct audio_format *format,
 	   G_GNUC_UNUSED int volume)
 {
+	return true;
 }
 
 struct decoder {