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.
