More floating

Found another case.  Kneel a flying unit on a destructible roof and shoot out the floor tile.  Unit carries on kneeling and _floating is not set to true.

![screen002](https://f.cloud.github.com/assets/1824834/1751011/0b6fa354-65c5-11e3-961b-52782572743e.png)

Tested against other cases in bug413 and nothing broke to my knowledge.