Parse errors leaking from XML::Reader

[straight-shoota]

When reading an invalid document with XML::Reader, parser errors remain stored and are falsely attributed to subsequent parses.

Example:

require "xml"

reader = XML::Reader.new(%(</foo>)) # Invalid XML
reader.read
reader.expand?

xml = XML.parse(%(<people></people>))
xml.errors # => [#<XML::Error:StartTag: invalid element name>]

A reason for that is that the XML::Reader instance is never closed to clean up. It does not even provide a method for doing that.

And even then there's the problem of global error state in libxml2. If multiple XML::Reader instance are intermingled (or an XML::Reader with XML.read), errors would be totally messed up.

A simple solution for the problem demonstrated above would be to reset errors in XML.parse and its sister methods. These methods are self-contained, so the error state could even be reset to what it was before. This would preserver functionality when called within an outer XML::Reader "session".
There's no solution for intermingled XML::Reader instances though.

This bug was discovered through spec randomization in #12541 (https://github.com/crystal-lang/crystal/actions/runs/3303805339/jobs/5452171721).