zerg multiplier resets for players after mine closes

src/engine/gameManager/SimulationManager.java

@@ -201,7 +201,7 @@ public enum SimulationManager {
             city.onEnter();
         }
         for(Mine mine : Mine.getMines()){
-            if(mine != null)
+            if(mine != null && mine.isActive)
                 mine.onEnter();
         }
         _cityPulseTime = System.currentTimeMillis() + CITY_PULSE;