gh-143008: Fix Null pointer dereferences in TextIOWrapper underlying stream access...

gh-143008: Fix Null pointer dereferences in TextIOWrapper underlying stream access (#145957)

TextIOWrapper keeps its underlying stream in a member called
`self->buffer`. That stream can be detached by user code, such as custom
`.flush` implementations resulting in `self->buffer` being set to NULL.
The implementation often checked at the start of functions if
`self->buffer` is in a good state, but did not always recheck after
other Python code was called which could modify `self->buffer`.

The cases which need to be re-checked are hard to spot so rather than
rely on reviewer effort create better safety by making all self->buffer
access go through helper functions.

Thank you yihong0618 for the test, NEWS and initial implementation in
gh-143041.

Co-authored-by: yihong0618 <zouzou0208@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>