Add bounds checking before indexing vecvecTempMemory

[softins]

Short description of changes

Bug fix to add bounds checking to buffer initialisation when changing size of the server jitter buffer.

Some of the added tests might not be strictly necessary or reached, but they guarantee the bounds are checked.

CHANGELOG: Server: fix occasional crash seen when clients < 3.6 change jitter buffer size.

Context: Fixes an issue?

Fixes #3747

This has been raised against main, and it should be merged there first. We should then raise a branch for 3.12.2, starting at release/3_12_1 (6953c57) and do a 3.12.2 release by cherry-picking the commit from main.

Does this change need documentation? What needs to be documented and how?

No, bug fix only

Status of this Pull Request

Ready and tested

What is missing until this pull request can be merged?

Review

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

[ann0see]

Let's get this in. Thanks!

[ann0see]

Release tagged. I hope it's correct this way.

Android doesn't build - but that's very likely unrelated. We should retry later.