br/streamhelper: fix panic while removing task (#50869)

[YuJuncen]

This is an automated cherry-pick of #50869

What problem does this PR solve?

Issue Number: close #50839

Problem Summary:

What changed and how does it work?

This PR will do the nil check over the checkpoints.
This PR also will catch the panic generated during the subscription handler.
And some locking are acquired during handling incoming tasks.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fixed a bug that may make TiDB panic when stopping a log backup task.

[tiprow]

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-6.5-20231229-v6.5.6@7a0b4df). Click here to learn what that means.
The diff coverage is 79.4117%.

Additional details and impacted files

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BornChanger, Leavrth

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-18 05:26:49.900881306 +0000 UTC m=+162098.648504402: ☑️ agreed by BornChanger.
• 2024-02-18 05:28:04.36085252 +0000 UTC m=+162173.108475632: ☑️ agreed by Leavrth.