[Python] KeyValueMetadata.key() segfaults on an out-of-range index #51019

Describe the bug, including details regarding any error messages, version, and platform.

Summary

KeyValueMetadata.key(0) and KeyValueMetadata.value(0) cause a segmentation fault when called on an empty KeyValueMetadata object.

Versions

pyarrow 25.0.0, CPython 3.12.3, Ubuntu 24.04 x86_64, glibc 2.39.

Reproducer

Each call below reproduces independently.

import pyarrow as pa
pa.KeyValueMetadata().key(0)

import pyarrow as pa
pa.KeyValueMetadata().value(0)

Running either call produces:

Segmentation fault (core dumped)

An UBSan source build reports an out-of-range std::vector<std::string>::operator[] access from src/arrow/util/key_value_metadata.cc:169 or :175.

This was found while fuzzing Python C extension modules for a small research project.
It looks like a bug to me, but I would appreciate a check.
I would expect IndexError rather than an unchecked native vector access.

Component(s)

Python