validateCheckConstraint() shouldn't try to access the storage for
a partitioned table, because it no longer has any.  Creating a
_RETURN table on a partitioned table shouldn't be allowed, both
because there's no value in it and because trying to do so would
involve a validation scan against its nonexistent storage.

Amit Langote, reviewed by Tom Lane.  Regression test outputs
updated to pass by me.

Discussion: http://postgr.es/m/e5c3cbd3-1551-d6f8-c9e2-51777d632fd2@lab.ntt.co.jp