addWall() fails at the edge of the map

[sbj42]

Adding a wall at the very edge of the map fails. For instance:

const fovMap = new FieldOfViewMap(2, 2);
fovMap.addWall(0, 0, geom.Direction.NORTH);
fovMap.getWall(0, 0, geom.Direction.NORTH); // <-- returns false

addWall() is avoiding putting an opposing wall out of bounds, but in doing so it fails to put the requested in-bounds wall.

Walls on the edge of the map aren't of much use to the field-of-view algorithm, but in case someone relies on FieldOfViewMap to save wall positions, it should still remember them.