executor: fix query panic on INFORMATION_SCHEMA.CLUSTER_SLOW_QUERY with some slow query files (#54325)

[ti-chi-bot]

This is an automated cherry-pick of #54325

What problem does this PR solve?

Issue Number: close #54324

Problem Summary:

What changed and how does it work?

The old code does not handle all the cases well.
if i >= len(lines)-1 should be a typo, it should be something like if i >= len(chars)-1
And the simplified logic is just the same as this PR.

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

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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@ti-chi-bot: 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	a38b2b4	link	true	/test mysql-test

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release-7.5@431781f). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #54711   +/-   ##
================================================
  Coverage               ?   72.0181%           
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  Files                  ?       1412           
  Lines                  ?     411353           
  Branches               ?          0           
================================================
  Hits                   ?     296249           
  Misses                 ?      95218           
  Partials               ?      19886           

Flag	Coverage Δ
unit	72.0181% <100.0000%> (?)

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

Components	Coverage Δ
dumpling	53.9913% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.5252% <0.0000%> (?)