pgsql: Fix bug in original implementation of xmlserialize(): if user

Log Message:
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Fix bug in original implementation of xmlserialize(): if user specifies
a target type that isn't acceptable, the code failed to raise the proper
error. The result instead was to return a NULL expression tree, which
in a quick test led to a 'cache lookup failed for type 0' error later.

Patch 8.3 only --- I fixed this in HEAD as part of recent locations patch.

Tags:
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REL8_3_STABLE

Modified Files:
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pgsql/src/backend/parser:
parse_expr.c (r1.226 -> r1.226.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c?r1=1.226&r2=1.226.2.1)