PyUnstable_Eval_RequestCodeExtraIndex freefunc may be called with a null pointer

Bug report

Bug description:

The PyUnstable_Code_SetExtra path checks for the previous value being null but code_dealloc does not.

This can lead to freefunc being called with null either through the extra value being explicitly set to null or through a value being set on an extra value with a later index, without a value being set on an earlier index.

This doesn't match the documentation, which says that freefunc is not called for null values.

CPython versions tested on:

3.11, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-122492

从 Objects/codeobject.c 中的 PyUnstable_Code_SetExtra 检查以及报告中链接的 code_dealloc 路径开始。将这两条路径与文档中说明的 null 值行为进行比较，然后检查已链接的 PR gh-122492，了解已经在进行的工作及其验证方法。