Mapgen fix: lab and pawn shop

Fixes #7545

Some labs are created from a fixed map (the one with the water filled room, the cargo crates, analyzer, ...). Those fixed maps have no border one the left and on the top, and a simple wall plus doors in the middle at the bottom and at the right side.

After the map has been initialized with those fixed maps, walls are added to those sides that *don't* connect to other lab overmap terrains (to seal the lab, I assume). This hides the plain rock of the neighboring overmap and removes the doors that would lead into plain rock.

The problem is that the fixed maps have furniture (fridge, cargo crates) directly on the left side (x=0) and the wall terrain that gets added here would override that.

The changes prevent the walls where they would override the furniture. The rock of the neighboring overmap tile is visible there.

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> This bug also shows up with Pawn shops.

Fixed: removing all furniture in the office (setting all terrain to floor and removing items was already done).