Fix: Bug report #241 - Avenger fly without elerium can take off

Bug report 241: http://openxcom.org/bugs/openxcom/issues/241

Now, crafts can't fly without fuel.

Also resolved following issue:
1. Select interceptor and send to way point.
2. Choose patrol mode.
3. Wait until this interceptor will have low of fuel and will start return to base.
4. Select this interceptor via "Intercept" menu and send to other way point (yes, it is possible).
5. Wait until this interceptor runs out of fuel.
6. Congratulation! You have interceptor, which can fly forever.