decoder_control: merge next_song and current_song

These two variables are redundant, we need only one of them.

src/decoder_thread.c

@@ -268,9 +268,11 @@ decoder_run_song(struct decoder_control *dc,
 static void
 decoder_run(struct decoder_control *dc)
 {
-	const struct song *song = dc->next_song;
+	const struct song *song = dc->song;
 	char *uri;
 
+	assert(song != NULL);
+
 	if (song_is_file(song))
 		uri = map_song_fs(song);
 	else
@@ -281,7 +283,6 @@ decoder_run(struct decoder_control *dc)
 		return;
 	}
 
-	dc->current_song = dc->next_song; /* NEED LOCK */
 	decoder_run_song(dc, song, uri);
 	g_free(uri);