db/LightSong: make Tag a reference

This enforces the "not nullptr" rule.

src/queue/PlaylistUpdate.cxx

@@ -50,7 +50,7 @@ UpdatePlaylistSong(const Database &db, DetachedSong &song)
 	}
 
 	song.SetLastModified(original->mtime);
-	song.SetTag(*original->tag);
+	song.SetTag(original->tag);
 
 	db.ReturnSong(original);
 	return true;