8322511: [11u] JfrCheckpointThreadClosure::do_thread crashes when fetching thread_id

Reviewed-by: phh

Author: jia-wei-tang

src/hotspot/share/jfr/recorder/checkpoint/types/jfrType.cpp

@@ -101,7 +101,7 @@ void JfrCheckpointThreadClosure::do_thread(Thread* t) {
   if (t->is_Java_thread()) {
     JavaThread* const jt = (JavaThread*)t;
     _writer.write(jt->name());
-    _writer.write(java_lang_Thread::thread_id(jt->threadObj()));
+    _writer.write(jt->threadObj() != NULL ? java_lang_Thread::thread_id(jt->threadObj()) : 0);
     _writer.write(JfrThreadGroup::thread_group_id(jt, _curthread));
     // since we are iterating threads during a safepoint, also issue notification
     JfrJavaEventWriter::notify(jt);