Physics bug - The invisi-ladder jump

[VernJensen]

I've identified the problem and repeated it myself quite a bit now, so I should be able to fix it.

The ladder-climbing code is some of the most complicated movement code in the game. I remember spending nearly 2 weeks just to get the initial code written the first time around, not to mention bug fixes along the way afterward. So it's tricky, but I'm sure I'll manage.  

Thanks, Rohful, for sending me such a nice package to demonstrate it so well!