Bug fix for Loon

To prevent occasional crash when a resource finishes getting mined, apply this change to Game/Code/missions.py:

--- missions.py.orig    2013-09-07 17:51:27.000000000 +1200
+++ missions.py    2013-09-14 09:41:07.000000000 +1200

@@ -45,7 +45,7 @@
             return target.resources
    
     def units_removed(self, unit, units):
-        if self.target in units:
+        if self.get_target_unit() in units:
             self.target = None
             if self.next_mission:
                 self.next_mission.units_removed(unit, units)