demux: parent chapter metadata to the chapter list

demux_disc replaces the chapter list at runtime. Parent each chapter's
tags to the list that contains them, so a replaced list keeps owning
its metadata instead of accumulating it on the demuxer.

demux/demux.c

@@ -4341,6 +4341,7 @@ int demuxer_add_chapter(demuxer_t *demuxer, char *name,
     };
     mp_tags_set_str(new.metadata, "TITLE", name);
     MP_TARRAY_APPEND(demuxer, demuxer->chapters, demuxer->num_chapters, new);
+    talloc_steal(demuxer->chapters, new.metadata);
     return demuxer->num_chapters - 1;
 }