8236487: JFR Recorder Thread crashed due to "assert(_chunkwriter.is_v…

…alid()) failed: invariant"

Reviewed-by: mgronlun, mseledtsov

src/hotspot/share/jfr/recorder/service/jfrRecorderService.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2016, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -671,7 +671,9 @@ void JfrRecorderService::invoke_flush() {
 
 void JfrRecorderService::flushpoint() {
   MutexLocker lock(JfrStream_lock, Mutex::_no_safepoint_check_flag);
-  invoke_flush();
+  if (_chunkwriter.is_valid()) {
+    invoke_flush();
+  }
 }
 
 void JfrRecorderService::process_full_buffers() {