player: assert that there was no previous "next song chunk"

When assigning the next_song_chunk variable, it must have been empty.
If not, there may be 3 songs overlapping in the music pipe.

src/player_thread.c

@@ -357,6 +357,7 @@ static void do_play(void)
 			/* the decoder has finished the current song;
 			   make it decode the next song */
 			assert(pc.next_song != NULL);
+			assert(player.next_song_chunk == -1);
 
 			player.queued = false;
 			player.next_song_chunk = music_pipe_tail_index();