expression: fix overflow panic in conv

[gengliqi]

What is changed and how it works?

Issue Number: Close #16969

What's Changed:

fix overflow panic in `conv`

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

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

Side effects

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

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• breezewish
• overvenus

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[gengliqi]

/retest

[gengliqi]

/merge

[ti-chi-bot]

@gengliqi: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

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

Commit hash: 372b54e

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #16978.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-5.4: #16979.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #16980.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #16981.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #16982.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #16983.