pgsql: Add checks to verify that a plpgsql function returning a rowtype

Log Message:
-----------
Add checks to verify that a plpgsql function returning a rowtype is actually
returning the rowtype it's supposed to return. Per reports from David Niblett
and Michael Fuhr.

Modified Files:
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pgsql/src/pl/plpgsql/src:
pl_exec.c (r1.158 -> r1.159)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.158&r2=1.159)