pgsql: Add some defenses against functions declared to return set that

Log Message:
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Add some defenses against functions declared to return set that don't
actually follow the protocol; per example from Kris Jurka.

Modified Files:
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pgsql/src/backend/executor:
execQual.c (r1.177 -> r1.178)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execQual.c.diff?r1=1.177&r2=1.178)