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

從 Objects/codeobject.c 中的 PyUnstable_Code_SetExtra 檢查以及報告中連結的 code_dealloc 路徑開始。將這兩條路徑與文件中說明的 null 值行為進行比較，然後檢查已連結的 PR gh-122492，了解已經在進行的工作及其驗證方法。