pgsql: Back-patch changes of 2009-05-13 in xml.c's memory management.

Log Message:
-----------
Back-patch changes of 2009-05-13 in xml.c's memory management.

I was afraid to do this when these changes were first made, but now that
8.4 has seen some field use it should be all right to back-patch.  These
changes are really quite necessary in order to give xml.c any hope of
co-existing with loadable modules that also wish to use libxml2.

Tags:
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REL8_3_STABLE

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xact.c (r1.257.2.6 -> r1.257.2.7)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c?r1=1.257.2.6&r2=1.257.2.7)
    pgsql/src/backend/utils/adt:
        xml.c (r1.68.2.11 -> r1.68.2.12)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.68.2.11&r2=1.68.2.12)
    pgsql/src/include/utils:
        xml.h (r1.23 -> r1.23.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/xml.h?r1=1.23&r2=1.23.2.1)