Fix: Losing in the assault on alien base.

Was fixed a situation, when player loses all soldiers, but alien base marked as destroyed.
http://openxcom.org/bugs/openxcom/issues/385

![screen088x50](https://f.cloud.github.com/assets/3616568/1538228/e21df906-4cdd-11e3-870e-5fb7e2e14d53.png)