Updated ThreadSafeLazyLoadedivoryTower

Added null check in private constructor to prevent instantiating by Reflection call

singleton/src/main/java/com/iluwatar/singleton/ThreadSafeLazyLoadedIvoryTower.java

@@ -33,7 +33,12 @@ public final class ThreadSafeLazyLoadedIvoryTower {
 
   private static ThreadSafeLazyLoadedIvoryTower instance;
 
-  private ThreadSafeLazyLoadedIvoryTower() {}
+  private ThreadSafeLazyLoadedIvoryTower() {
+  // to prevent instantiating by Reflection call
+    if (instance != null) {
+      throw new IllegalStateException("Already initialized.");
+    }
+  }
 
   /**
    * The instance gets created only when it is called for first time. Lazy-loading