Update GDWheelPolicy.java

A fix for null pointer in migrate.
changed the condition of the if statment as to not cause a null pointer exception by trying to access wheel at index that is out of bounds.

simulator/src/main/java/com/github/benmanes/caffeine/cache/simulator/policy/greedy_dual/GDWheelPolicy.java

@@ -122,7 +122,7 @@ private void migrate(int level) {
     clockHand[level] = hand;
 
     // if C[idx] has advanced a whole round back to 1, call migration(idx+1)
-    if ((hand == 0) && (level < wheel.length)) {
+    if ((hand == 0) && (level + 1 < wheel.length)) {
       migrate(level + 1);
     }