Fix create table error (#4630)

[ti-chi-bot]

This is an automated cherry-pick of #4630

What problem does this PR solve?

Issue Number: close #4596

Problem Summary:

What is changed and how it works?

make sure Flen >= 0 before append.

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

Documentation

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

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JaySon-Huang

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[Lloyd-Pottiger]

/run-integration-test

[Lloyd-Pottiger]

/cc @JaySon-Huang @flowbehappy

[JaySon-Huang]

close as no further patch version for the 6.0 branch