Fix segfault when no mixer channels given in ini

src/audio_mixer.c

@@ -589,6 +589,7 @@ void mixer_init(void)
 		mixers[0].name = strdup("Music");
 		mixers[1].name = strdup("Sound");
 		mixers[2].name = strdup("Master");
+		master = &mixers[2];
 	} else {
 		// NOTE: Older games don't have an explicit master channel.
 		//       We add an extra mixer in this case for consistency.