arrow

GH-44526: [C++][Acero] Fix crash when thread in asof_join is not running (#44584)

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### What changes are included in this PR?

I add checking joinable before join process_thread. It will prevent
exeptions in case when plan is invalid and asof_join never_starts.
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* GitHub Issue: #44526

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Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Co-authored-by: Rossi Sun <zanmato1984@gmail.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>