faulthandler_get_fileno returns borrowed sys.stderr without incref, leading to unintended decref

[AZero13]

Bug report

Bug description:

faulthandler_get_fileno fetches sys.stderr with PySys_GetAttr (borrowed ref) when file is NULL/Py_None, then hands it back to callers who Py_XDECREF it. Because we never incref the borrowed ref, we end up decref-ing sys.stderr (or Py_None on the error path), potentially dropping its refcount unexpectedly.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-142442: Increase the borrowed ref from faulthandler_get_fileno when file is not None instead of decreasing when it is None #142443