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

Log Message:
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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.

Tags:
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REL8_3_STABLE

Modified Files:
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pgsql/contrib/xml2:
Makefile (r1.11.2.1 -> r1.11.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/Makefile?r1=1.11.2.1&r2=1.11.2.2)
xpath.c (r1.16.2.2 -> r1.16.2.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xpath.c?r1=1.16.2.2&r2=1.16.2.3)
xslt_proc.c (r1.9.2.1 -> r1.9.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xslt_proc.c?r1=1.9.2.1&r2=1.9.2.2)

Added Files:
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pgsql/contrib/xml2/expected:
xml2.out (r1.1.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/expected/xml2.out?rev=1.1.4.1&content-type=text/x-cvsweb-markup)
pgsql/contrib/xml2/sql:
xml2.sql (r1.1.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/sql/xml2.sql?rev=1.1.4.1&content-type=text/x-cvsweb-markup)