Minor Code Update: disappearing Air Units fix

An occasional and unfrequent bug leading to the disappearance of air units after battle, during the return to base phase, has been found and fixed. Planes shall no longer vanish if they can find an eligible airfield within range.

They will still disappear when none is rachable, either because the said arifield(s) is/are full or is/are out of range of the plane(s).