[3.13] gh-148441: Avoid integer overflow in Expat's CharacterDataHandler (GH-148904...

[3.13] gh-148441: Avoid integer overflow in Expat's CharacterDataHandler (GH-148904) (#149637)

* gh-148441: Avoid integer overflow in Expat's CharacterDataHandler (GH-148904)
(cherry picked from commit bc1be4f6174086b4a46e3fe656552f5bb4e6e7b2)

Co-authored-by: ByteFlow <fakeshadow1337@gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
* Apply suggestion from @picnixz

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
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Co-authored-by: ByteFlow <fakeshadow1337@gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>