message: Clear CWorker->m_ptsk field on exception.
  If an error occurs, a worker is no longer executing the given task. This
  was causing problems later, when the Task object had already been
  destroyed, but m_ptsk was left dangling.
diff: @@ -124,9 +124,18 @@ CWorker::Execute(CTask *ptsk)
  #endif // GPOS_DEBUG
  m_ptsk = ptsk;
  + GPOS_TRY
  + {
  m_ptsk->Execute();
  m_ptsk = NULL;
  }
  + GPOS_CATCH_EX(ex)
  + {
  + m_ptsk = NULL;
  + GPOS_RETHROW(ex);
  + }
  + GPOS_CATCH_END;
  +}
  //---------------------------------------------------------------------------
  