vtkThreadedCallbackQueue, fixing race condition

The boolean member `Destroying` needs to be atomic because there is one
location in the code that uses it without mutex
protectionThreadedCallbackQueue, fixing race condition

The boolean member `Destroying` needs to be atomic because there is one
location in the code that uses it without mutex protection

Parallel/Core/vtkThreadedCallbackQueue.cxx

@@ -79,8 +79,8 @@ private:
    */
   bool OnHold() const
   {
-    return *this->ThreadIndex < this->Queue->NumberOfThreads && !this->Queue->Destroying &&
-      this->Queue->InvokerQueue.empty();
+    return *this->ThreadIndex < this->Queue->NumberOfThreads &&
+      !this->Queue->Destroying.load(std::memory_order_acquire) && this->Queue->InvokerQueue.empty();
   }
 
   /**

Parallel/Core/vtkThreadedCallbackQueue.h

@@ -477,7 +477,7 @@ private:
    * This atomic boolean is false until destruction. It is then used by the workers
    * so they know that they need to terminate when the queue is empty.
    */
-  bool Destroying = false;
+  std::atomic_bool Destroying{ false };
 
   /**
    * Number of allocated threads. Allocated threads are not necessarily running.