gh-143378: Fix use-after-free when BytesIO is concurrently mutated during write operations (GH-143408)

PyObject_GetBuffer() can execute user code (e.g. via __buffer__), which may
close or otherwise mutate a BytesIO object while write() or writelines()
is in progress. This could invalidate the internal buffer and lead to a
use-after-free.

Ensure that PyObject_GetBuffer() is called before validation checks.