pgsql: Fix map_sql_typecoll_to_xmlschema_types() to not fail on dropped

Log Message:
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Fix map_sql_typecoll_to_xmlschema_types() to not fail on dropped
columns, per my gripe earlier today. Make it look a bit less like
someone's first effort at backend coding.

Modified Files:
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pgsql/src/backend/utils/adt:
xml.c (r1.45 -> r1.46)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.45&r2=1.46)