null check fix

src/engine/objects/Mob.java

@@ -1711,7 +1711,10 @@ public class Mob extends AbstractIntelligenceAgent {
                 this.behaviourType = MobBehaviourType.GuardCaptain;
                 this.spawnTime = 60 * 15;
                 this.isPlayerGuard = true;
+                if(ZoneManager.getCityAtLocation(this.building.getLoc()) != null)
                     this.guardedCity = ZoneManager.getCityAtLocation(this.building.getLoc());
+                else
+                    this.guardedCity = null;
             }
 
             // Load AI for wall archers