Back-patch today's memory management fixups in contrib/xml2.

Prior to 8.3, these changes are not critical for compatibility with core
Postgres, since core had no libxml2 calls then.  However there is still
a risk if contrib/xml2 is used along with libxml2 functionality in Perl
or other loadable modules.  So back-patch to all versions.

Also back-patch addition of regression tests.  I'm not sure how many of
the cases are interesting without the interaction with core xml code,
but a silly regression test is still better than none at all.

contrib/xml2/sql/xml2.sql

@@ -0,0 +1,94 @@
+--
+-- first, define the functions.  Turn off echoing so that expected file
+-- does not depend on contents of pgxml.sql.
+--
+SET client_min_messages = warning;
+\set ECHO none
+\i pgxml.sql
+\set ECHO all
+RESET client_min_messages;
+
+select xslt_process( 
+'<table xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+
+<row>
+  <x>1</x>
+</row>
+
+<row>
+  <x>2</x>
+</row>
+
+<row>
+  <x>3</x>
+</row>
+
+<row>
+  <x>4</x>
+</row>
+
+<row>
+  <x>5</x>
+</row>
+
+</table>'::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);
+
+CREATE TABLE xpath_test (id integer NOT NULL, t text);
+INSERT INTO xpath_test VALUES (1, '<doc><int>1</int></doc>');
+SELECT * FROM xpath_table('id', 't', 'xpath_test', '/doc/int', 'true')
+as t(id int4);
+SELECT * FROM xpath_table('id', 't', 'xpath_test', '/doc/int', 'true')
+as t(id int4, doc int4);
+
+create table articles (article_id integer, article_xml text, date_entered date);
+insert into articles (article_id, article_xml, date_entered)
+values (2, '<article><author>test</author><pages>37</pages></article>', now());
+SELECT * FROM
+xpath_table('article_id',
+            'article_xml',
+            'articles',
+            '/article/author|/article/pages|/article/title',
+            'date_entered > ''2003-01-01'' ')
+AS t(article_id integer, author text, page_count integer, title text);
+
+-- this used to fail when invoked a second time
+select xslt_process('<aaa/>',$$<xsl:stylesheet version="1.0"
+xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+<xsl:template match="@*|node()">
+      <xsl:copy>
+         <xsl:apply-templates select="@*|node()"/>
+      </xsl:copy>
+   </xsl:template>
+</xsl:stylesheet>$$);
+
+select xslt_process('<aaa/>',$$<xsl:stylesheet version="1.0"
+xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+<xsl:template match="@*|node()">
+      <xsl:copy>
+         <xsl:apply-templates select="@*|node()"/>
+      </xsl:copy>
+   </xsl:template>
+</xsl:stylesheet>$$);
+
+create table t1 (id integer, xml_data text);
+insert into t1 (id, xml_data)
+values
+(1, '<attributes><attribute name="attr_1">Some
+Value</attribute></attributes>');
+
+create index idx_xpath on t1 ( xpath_string
+('/attributes/attribute[@name="attr_1"]/text()', xml_data));