Fix current_thread_index for masterless arena(1,0)

[pavelkumbrasev]

Description

A masterless arena(max concurrency = 1, master threads = 0) values might return unexpected tbb::this_task_arena::current_thread_index(), not 0 but 1. That happens because arena preserve additional slot for arenas with max_concurrency == 1 for correct support of enqueue tasks (to ensure that there is a slot for a worker in case of enqueue task submit). However, it is not the case for arena(max concurrency = 1, master threads = 0) because there is a slot for worker thread.

Fixes # - issue number(s) if exists

• - git commit message contains an appropriate signed-off-by string (see CONTRIBUTING.md for details)

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• bug fix - change that fixes an issue
• new feature - change that adds functionality
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Tests

• added - required for new features and some bug fixes
• not needed

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• needs to be updated
• not needed

Breaks backward compatibility

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Other information

[dnmokhov]

LGTM

[isaevil]

LGTM