If the CHANGE_SLAB_TYPE script command is used to change a room slab into another room slab, it would leave the old room present in the slab information.

Function do_slab_efficiency_alteration would do this:

struct SlabAttr* slbattr = get_slab_attrs(slb);
        if (slbattr->category == SlbAtCtg_RoomInterior)
        {
            struct Room* room = slab_room_get(sslb_x, sslb_y);

And find a room of type NULL, causing several log errors and related issues, possibly also player notifications when the room was destroyed.
This PR cleans that up.

To reproduce the original issue:

1. Run this map: map30117.zip
2. Press ~ to see logging
3. Wait for 140 gameturns to see rooms get replaced
   -> Notice a ! message that says a room is destroyed, with the eye at pos (0,0).