message: only use aggregator comm if it is active (created). Also change file name for the test
diff: @@ -386,9 +386,13 @@ void BP4Writer::InitBPBuffer()
  m_BP4Serializer.m_PreDataFileLength =
  m_FileDataManager.GetFileSize(0);
  }
  + if (m_BP4Serializer.m_Aggregator.m_IsActive)
  + {
  + // only use aggregator comm if it is active (created)
  m_BP4Serializer.m_PreDataFileLength =
  m_BP4Serializer.m_Aggregator.m_Comm.BroadcastValue(
  m_BP4Serializer.m_PreDataFileLength);
  + }
  if (m_BP4Serializer.m_RankMPI == 0)
  {
  