[Fixed] Monsters can not walk onto/off 3D floors over portal

[boris]

Looks like non-flying monsters can not walk onto or off of a 3D floor that's over a sector portal.

Tested with g3.8pre-37-g7c8cdf80d. Example map is attached.

[Graf Zahl]

Fixed. Although with this code you can never be sure if there isn't some gotcha to look out for.