executor: fix data race in `mem_reader.go`

[Defined2014]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

What changed and how does it work?

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.

Side effects

• [ ] Performance regression: Consumes more CPU
• [ ] Performance regression: Consumes more Memory
• [ ] Breaking backward compatibility

Documentation

• [ ] Affects user behaviors
• [ ] Contains syntax changes
• [ ] Contains variable changes
• [ ] Contains experimental features
• [ ] Changes MySQL compatibility

Release note

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

None

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[Defined2014]

/retest

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@Defined2014: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

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/retest

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@Defined2014: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/mysql-test	742284cf30fa45d5fa7ab0be5c0443340020632c	link	true	/test mysql-test

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