Paper over a layout_big_octagon issue (#9174).

Basically, this occurs when we get an entire row / column of solid rock
and replace_first tries to place a stair along that row/column and fails
because there's no open space there. If that happens, the layout is
almost certainly going to veto anyway (you'll only see solid
rows/columns when the map is too small to place properly), so don't
require that the check succeed.