pgsql: Reactivate libxml memory management via palloc, now that I think

Log Message:
-----------
Reactivate libxml memory management via palloc, now that I think I've
classified the conditions under which this is safe to do (see source
code comment).

Modified Files:
--------------
pgsql/src/backend/utils/adt:
xml.c (r1.23 -> r1.24)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.23&r2=1.24)