Incorrect handling of start and end values in codecs.xmlcharrefreplace_errors handler

[picnixz]

Crash report

What happened?

./python -c "import codecs; codecs.xmlcharrefreplace_errors(UnicodeEncodeError('bad', '', 0, 1, 'reason'))"
python: ./Include/cpython/unicodeobject.h:339: PyUnicode_READ_CHAR: Assertion `index >= 0' failed.
Aborted (core dumped)

See #123378 for the root cause. Since we are still wondering how to fix the getters and setters, I suggest we first fix the crash by adding the checks inside at the handler's level (for now). I'm not sure if the handler itself is handling corner cases correctly as well.

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Note: not able to work on it today so I'll do it tomorrow.