[Wisp] Fix wrong assert.

Summary: WispTask could be moved between carriers during
yield, so should refetch current task.

Test Plan: com/alibaba/wisp

Reviewed-by: yulei

Issue:
dragonwell-project#140

src/java.base/share/classes/com/alibaba/wisp/engine/WispCarrier.java

@@ -229,7 +229,7 @@ final void schedule() {
     }
 
     private void checkAndDispatchShutdown() {
-        assert WispCarrier.current() == this;
+        WispTask current = WispCarrier.current().getCurrentTask();
         if ((engine.hasBeenShutdown
                 || (current.inDestoryedGroup() && current.inheritedFromNonRootContainer()))
                 && !WispTask.SHUTDOWN_TASK_NAME.equals(current.getName())