[Freeciv-Dev] (PR#12850) AI Tries to Reinforce Lost Cities

["Benedict Adamson" <badamson@xxxxxxxxxxx>]

<URL: http://bugs.freeciv.org/Ticket/Display.html?id=12850 >

The function ai_military_defend prefers trying to defend a city to which 
a defender ahas already been assigned, as recorded in punit->ai.charge. 
However, in between being assigned to the defense of that city and the 
execution of ai_military_defend the city might have been captured by an 
enemy.

ai_military_defend should check for lost cities, and clear the 
assignment of a unit if its assignment has been lost.