[Bugfix][V1][Minor] Fix shutting_down flag checking in V1 MultiprocExecutor

[bigPYJ1151]

I noticed the checking for shutting_down flag in V1 MultiprocExecutor will always be False, and it ignores _ensure_worker_termination for terminating worker processes gracefully in some situations.

This PR fixed it.

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[bigPYJ1151]

Hi @tlrmchlsmth, can you help to review this fix? I think you are the author of the v1 MP executor, thanks :)

[tlrmchlsmth]

Nice catch, thanks!