Add volatile in double-checked locking field in ClusterBuilderSlot (#466)

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/clusterbuilder/ClusterBuilderSlot.java

@@ -70,7 +70,7 @@ public class ClusterBuilderSlot extends AbstractLinkedProcessorSlot<DefaultNode>
 
     private static final Object lock = new Object();
 
-    private ClusterNode clusterNode = null;
+    private volatile ClusterNode clusterNode = null;
 
     @Override
     public void entry(Context context, ResourceWrapper resourceWrapper, DefaultNode node, int count,