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_1_STABLE

Modified Files:
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    pgsql/contrib/xml2:
        Makefile (r1.6.2.2 -> r1.6.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/Makefile?r1=1.6.2.2&r2=1.6.2.3)
        xpath.c (r1.8.2.2 -> r1.8.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xpath.c?r1=1.8.2.2&r2=1.8.2.3)
        xslt_proc.c (r1.4.2.1 -> r1.4.2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xslt_proc.c?r1=1.4.2.1&r2=1.4.2.2)

Added Files:
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    pgsql/contrib/xml2/expected:
        xml2.out (r1.1.8.1)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/expected/xml2.out?rev=1.1.8.1&content-type=text/x-cvsweb-markup)
    pgsql/contrib/xml2/sql:
        xml2.sql (r1.1.8.1)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/sql/xml2.sql?rev=1.1.8.1&content-type=text/x-cvsweb-markup)