[issue32827] Fix incorrect usage of _PyUnicodeWriter_Prepare()

[Xiang Zhang]

Xiang Zhang <angwerzx at 126.com> added the comment:

Nice. The only question I have is this is a bugfix or enhancement? Current code seems won't cause any problem but just allocates more memory than needed.

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stage: patch review -> commit review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32827>
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