fix github issue #955 - tile2bmp bounds failure

fix github issue #955 - tile2bmp bounds failure

Issue reported by argrath:  building with 'address sanitizing'
reported tile2bmp writing out of array bounds after the addition
of the generic object tiles.

'MAGICTILENO' in tile2bmp.c is extremely fragile.  It was already
inaccurate before the generic tiles, but had a big enough value to
handle the final row of tiles prior to that.

Fixes #955