MDEV-31566 Fix buffer overrun of column_json function

[tohghua]

See https://jira.mariadb.org/browse/MDEV-31566

Test case: the following sql should work fine.
select column_json(0x0402000A0000000300030023076A736E7375626A6563742E0005006C0027000200290002002B0002002D0002002F0002000C31000C3B000C4B000C51000F62006631663266336634663509E5A79AE8BF9CE6B48B0FE8819AE9809AE98791E6A1A5E5BA970537343530301031313634332F393634352F31313630300C080000000000EFBFBDEFBFBD192E);
select column_json(0x0402000900000003000300740C6A736E766F6C756D652E000900EFBFBD004300020045000200470003004A0004004E00050053000500580005005D000500620005000C67000C6A000C6D000C7000052C00051B00052C000CEFBFBD0007EFBFBD006638663966313070696332626F785F63626F785F67626F785F6B626F785F7666355F696402343402343402333241687474703A2F2F6F73732E68646238382E636F6D2F302F70686F746F2F30373865653765376336343634616236386130343833373333323636613532612E67696608302E303532323732244F1E00030180C106);

[anson1014]

Hi @tohghua, please follow the PR template for contributions

<!--
Thank you for contributing to the MariaDB Server repository!

You can help us review your changes faster by filling in this template <3

If you have any questions related to MariaDB or you just want to hang out and meet other community members, please join us on https://mariadb.zulipchat.com/ .
-->

<!--
If you've already identified a https://jira.mariadb.org/ issue that seems to track this bug/feature, please add its number below.
-->
- [x] *The Jira issue number for this PR is: MDEV-______*

<!--
An amazing description should answer some questions like:
1. What problem is the patch trying to solve?
2. If some output changed that is not visible in a test case, what was it looking like before the change and how it's looking with this patch applied?
3. Do you think this patch might introduce side-effects in other parts of the server?
-->
## Description
TODO: fill description here

## Release Notes
TODO: What should the release notes say about this change?
Include any changed system variables, status variables or behaviour. Optionally list any https://mariadb.com/kb/ pages that need changing.

## How can this PR be tested?

TODO: modify the automated test suite to verify that the PR causes MariaDB to behave as intended.
Consult the documentation on ["Writing good test cases"](https://mariadb.org/get-involved/getting-started-for-developers/writing-good-test-cases-mariadb-server).
<!--
In many cases, this will be as simple as modifying one `.test` and one `.result` file in the `mysql-test/` subdirectory.
Without automated tests, future regressions in the expected behavior can't be automatically detected and verified.
-->

If the changes are not amenable to automated testing, please explain why not and carefully describe how to test manually.

<!--
Tick one of the following boxes [x] to help us understand if the base branch for the PR is correct.
see [CODING_STANDARDS.md](https://github.com/MariaDB/server/blob/-/CODING_STANDARDS.md) for the latest versions.
-->
## Basing the PR against the correct MariaDB version
- [ ] *This is a new feature and the PR is based against the latest MariaDB development branch.*
- [ ] *This is a bug fix and the PR is based against the earliest maintained branch in which the bug can be reproduced.*

<!--
  All code merged into the MariaDB codebase must meet a quality standard and codying style.
  Maintainers are happy to point out inconsistencies but in order to speed up the review and merge process we ask you to check the CODING standards.
-->
## PR quality check
- [ ] I checked the [CODING_STANDARDS.md](https://github.com/MariaDB/server/blob/-/CODING_STANDARDS.md) file and my PR conforms to this where appropriate.
- [ ] For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

Furthermore, your commit message is very vague and does not describe what the issue you are fixing is, why you are fixing it or when it occurs. Please consider adding some more detail. Furthermore, you describe in the current PR description a test case that should pass with the addition of your new changes. Perhaps add an MTR test to verify this as well.

Here is some good documentation you can refer to for what I've mentioned above:

• Make sure your commit messages and content follow the following guidelines
• Writing good test cases for MariaDB server

[grooverdan]

Implementation looks good.

Thanks for signing the CLA.

The commit message will need to be improved and test case added for this to merge (back to 10.5 branch).

If you want use to take care of these details please let us know.

[grooverdan]

so we moved this because there could be up to 2 more characters. We could be over allocating, but I'm not sure it matters.

[tohghua]

Over allocating should be fine, it's just a buffer.

If you want us to take care of these details please let us know.
You're welcome :)

[grooverdan]

good.

[CLAassistant]

All committers have signed the CLA.

[grooverdan]

thank you for the fix @tohghua. Sorry I was slow applying your first fix.