Update Nullkiller.cpp

No more map-hack on 3rd difficulty-level. Only starting from the fourth.

AI/Nullkiller/Engine/Nullkiller.cpp

@@ -62,7 +62,7 @@ bool canUseOpenMap(std::shared_ptr<CCallback> cb, PlayerColor playerID)
 		return false;
 	}
 
-	return true;
+	return cb->getStartInfo()->difficulty >= 3;
 }
 
 void Nullkiller::init(std::shared_ptr<CCallback> cb, AIGateway * gateway)