Fix gathering infinite loop when the gatherer is out of the world.

Promoted units move out of the world but keep their current orders, which can trigger invalid states.
As cleanup, handle this case in GATHER.FINDINGNEWTARGET

A more general fix seems possible by putting the UnitAI in a default state.

Reported by: psypherium

Reviewed By: Angen

Fixes #5788

Differential Revision: https://code.wildfiregames.com/D2920