Fix #71592: External entity processing never fails

If the callback set via `xml_set_external_entity_ref_handler()` returns
a falsy value, parsing is supposed to stop and the error number set to
`XML_ERROR_EXTERNAL_ENTITY_HANDLING`.  This is already correctly done
by the libexpat binding, but the libxml2 binding ignores the return
value.  We fix this by calling `xmlStopParser()` which is available as
of libxml 2.1.0[1] (PHP-7.1 requires at least libxml 2.6.11 anyway),
and setting the desired `errNo` ourselves.

[1] <http://xmlsoft.org/news.html>

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