decoder_control: make the song objects const

src/decoder_control.h

@@ -72,8 +72,8 @@ struct decoder_control {
 	/** the format being sent to the music pipe */
 	struct audio_format out_audio_format;
 
-	struct song *current_song;
-	struct song *next_song;
+	const struct song *current_song;
+	const struct song *next_song;
 	float total_time;
 
 	/** the #music_chunk allocator */

src/decoder_thread.c

@@ -263,7 +263,7 @@ decoder_run_song(struct decoder_control *dc,
 static void
 decoder_run(struct decoder_control *dc)
 {
-	struct song *song = dc->next_song;
+	const struct song *song = dc->next_song;
 	char *uri;
 
 	if (song_is_file(song))

src/player_control.h

@@ -108,7 +108,7 @@ struct player_control {
 	float total_time;
 	float elapsed_time;
 	struct song *volatile next_song;
-	struct song *errored_song;
+	const struct song *errored_song;
 	volatile double seek_where;
 	float cross_fade_seconds;
 	double total_play_time;