[3.14] gh-157364: Fix use-after-free in io.TextIOWrapper during reentrant detach

[vstinner]

• Null pointer dereference in TextIOWrapper.truncate via re-entrant flush #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.

(cherry picked from commit db4b194)

• Data race: TextIOWrapper.detach() stores self->buffer = NULL non-atomically #154523: Fix data-race in TextIOWrapper.detach() (gh-154523: Fix data-race in TextIOWrapper.detach() #154565)

(cherry picked from commit 8440a6a)

• io.TextIOWrapper UAF triggered by readline() #157364: Fix use-after-free in io.TextIOWrapper during reentrant detach (gh-157364: Fix use-after-free in TextIOWrapper during reentrant detach #157370)

(cherry picked from commit e19dc47)

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(cherry picked from commit ea1312b)