Fixes#123
Make sure wallification doesn't go out of bounds when operating
near the map edge. The top and left edges were ok (although the
left edge could uselessly try to wallify unused column 0) but the
right and bottom ones weren't validating the map boundary. None
of the 3.6.x levels were affected.
I've done this a little differently from the suggested commit in
the pull request.
854155dfa9