message: Fix crash if app loads MPI module but makes no OSPRay API calls
  The threads in this case will not have been created, even though
  the MAML context is still created statically at startup/load.
diff: @@ -230,8 +230,12 @@ namespace maml {
  void Context::stop()
  {
  tasksAreRunning = false;
  + if (sendReceiveThread) {
  sendReceiveThread->stop();
  + }
  + if (processInboxThread) {
  processInboxThread->stop();
  + }
  flushRemainingMessages();
  }
  