br/streamhelper: fix panic while removing task

[YuJuncen]

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]

Hi @YuJuncen. Thanks for your PR.

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[codecov]

Codecov Report

Merging #50869 (3065aa8) into master (c0476a0) will increase coverage by 5.4023%.
Report is 96 commits behind head on master.
The diff coverage is 79.4117%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #50869        +/-   ##
================================================
+ Coverage   70.3642%   75.7665%   +5.4022%     
================================================
  Files          1456       1464         +8     
  Lines        420924     442599     +21675     
================================================
+ Hits         296180     335342     +39162     
+ Misses       104409      87203     -17206     
+ Partials      20335      20054       -281     

Flag	Coverage Δ
integration	50.2391% <29.4117%> (?)
unit	70.3082% <79.4117%> (-0.0376%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	54.0269% <ø> (ø)
parser	∅ <ø> (∅)
br	56.0622% <79.4117%> (+5.4930%)	⬆️

[hawkingrei]

/ok-to-test

[YuJuncen]

/retest-required

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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Needs approval from an approver in each of these files:

• br/OWNERS [BornChanger,Leavrth]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-02 06:02:33.126084619 +0000 UTC m=+1718194.690382324: ☑️ agreed by Leavrth.
• 2024-02-07 08:33:15.967430327 +0000 UTC m=+345721.534200202: ☑️ agreed by BornChanger.

[YuJuncen]

/run-cherry-picker

[YuJuncen]

/run-cherry-picker

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #51052.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #51053.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #51054.