Fix #81351: xml_parse may fail, but has no error code

The fix for bug #73151[1] cured the symptoms, but not the root cause,
namely xmlParse() must not be called recursively. Since that

Fix #81351: xml_parse may fail, but has no error code

The fix for bug #73151[1] cured the symptoms, but not the root cause,
namely xmlParse() must not be called recursively. Since that bugfix
also messed up the error handling, we basically revert it (but also
simplify the return), and then prevent calling the parser recursively.

[1] <https://github.com/php/php-src/pull/2166/commits/f2a8a8c068995a5d780882c556cedd53bce3827d>

Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>

Closes GH-7363.

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