statistics: fix panic occurs when stats cache inconsistency (#22465)

[ti-srebot]

cherry-pick #22465 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/22565

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/22565:release-4.0-f22ec62dd03f

---

What problem does this PR solve?

Issue Number: close #22464

Problem Summary:

1. panic occurs when stats cache inconsistency.
2. lock leak of loadStatsWorker which can make stats cache inconsistency.
   this problem solved by statistics: fix an unsafe lock operation and adjust some logging levels #20381

What is changed and how it works?

What's Changed:

1. make a check when calling mathutil.MaxInt64(ndv, col.NDV)

How it Works:

1. check key if exits in col map to avoid calling nil pointer

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
  No need.
• Need to cherry-pick to the release branch
  4.0.8 or above

Check List

Tests

• Unit test
  pass
• Integration test
  pass
• Manual test (add detailed scripts or steps below)
  pass
• No code

Side effects

• Performance regression
  may be little cpu cost
• Breaking backward compatibility

Release note

• Fix panic occurs when stats inconsistency

[ti-srebot]

/run-all-tests

[ti-srebot]

@ti-srebot you're already a collaborator in bot's repo.

[eurekaka]

LGTM

[eurekaka]

/run-all-tests

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• winoros

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[zhouqiang-cl]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 7a36064

[ti-chi-bot]

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[zhouqiang-cl]

/run-unit-tests