Properly handle invalid room references in savefiles. In all games, rs_write_room_reference() stored -1 for a nonexistent room, but rs_read_room_reference() did not check for out-of-bounds values, leading to pointers to rooms[-1], which sometimes caused crashes. rs_read_room_reference() has now been modified to use NULL instead. Some of the games required further changes to replace NULL with the pointer to the actual room. Others are capable of handling NULL for objects not in any room.