Backport of #31314.

PR summary

Add a NULL check for PySequence_GetItem in the multi_index setter to prevent
segmentation faults when getitem raises an exception.

As reported in:
https://gist.github.com/devdanzin/ccc2d9553ca1c90ab1835362ee21a40a#reproducer-3-nditermulti_index-segfault-with-raising-sequence
(discussed in #31046)

Reproduce error:

import numpy as np

class BadSequence:
    def __len__(self):
        return 2

    def __getitem__(self, i):
        if i == 1:
            raise RuntimeError("intentional error")
        return 0



arr = np.zeros((3, 4))
it = np.nditer(arr, flags=["multi_index"])

it.multi_index = BadSequence()

Actual behavior:

Segmentation fault         (core dumped)

Expected behavior:

RuntimeError: intentional error

First time committer introduction

AI Disclosure

The technical analysis, debugging, and code changes were performed and verified manually.