Fix move by waypoints not checking for sector changes

It seemed weird at first, but the polyobject sector (backsector) itself DOESN'T need checked (although I still am doing it for safety). Rather, the in-level sector just needs checked.

If someone manually modifies the polyobject sector though, then this bug can still occur... but this fixes it for the most common use-case where this can happen. I'll try to tackle the rarer cases in my next commit.