pgsql: Guard against possible double free during error escape from XML

Log Message:
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Guard against possible double free during error escape from XML
functions. Patch for the reported issue from Kris Jurka, some
other potential trouble spots plugged by Tom.

Modified Files:
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pgsql/src/backend/utils/adt:
xml.c (r1.48 -> r1.49)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.48&r2=1.49)