Add overflow checks before calling memory allocators

[picnixz]

Bug report

Bug description:

From capi-workgroup/decisions#50 (comment) and capi-workgroup/decisions#50 (comment).

Some calls to the PyMem_* API uses an arithmetic operation on the size, which could make the requested length overflow. The checks in the corresponding functions would not be relevant since the inputs already overflowed.

See #127686 (comment) for the future of this issue.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

[serhiy-storchaka]

In most cases there is such check (I myself added and reviewed miriads of checks). If it is forgotten in some case, it is a bug.

[picnixz]

I've found quite a lot of code that does not check them. There are some parts that do not need such checks because of some assumptions (e.g., when converting numbers to string, we likely won't have more digits than SIZE_MAX).

EDIT: I'll create a fresh branch without comments and with the minimal checks.