Prevent the :class:`~concurrent.futures.ProcessPoolExecutor` executor thread,
which remains running when :meth:`shutdown(wait=False)
<concurrent.futures.Executor.shutdown>`, from
attempting to adjust the pool's worker processes after the object state has already been reset during shutdown.
A combination of conditions, including a worker process having terminated abormally,
resulted in an exception and a potential hang when the still-running executor thread
attempted to replace dead workers within the pool.
