contrib/xml2 pfree bug

Didn't we just clean up a mess in our XML handling to do with memory
handlers? It looks like contrib/xml2 might have similar problems. Here's
the relevant part of a back trace from a core dump:

Program terminated with signal 11, Segmentation fault.
#0 0x000000000069300a in pfree ()
(gdb) bt
#0 0x000000000069300a in pfree ()
#1 0x000000356c42e0ee in xmlCleanupCharEncodingHandlers () from
/usr/lib64/libxml2.so.2
#2 0x000000356c436675 in xmlCleanupParser () from /usr/lib64/libxml2.so.2
#3 0x00002aaab072c5b6 in xslt_process () from
/bk/xxxx/dbinst-84/lib/postgresql/pgxml.so

this was generated from the following call (XML afficionados will
realise I was trying to pretty print the XML):

select xslt_process( cb_ob_invoice_xml(1,1)::text,
$$<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:output method="xml" indent="yes" />

<xsl:template match="*">
<xsl:copy>
<xsl:copy-of select="@*" />
<xsl:apply-templates />
</xsl:copy>
</xsl:template>
<xsl:template match="comment()|processing-instruction()">
<xsl:copy />
</xsl:template>

</xsl:stylesheet>
$$::text
);

cheers

andrew