Make regress function make_tuple_indirect() able to handle plain attributes

Make regress function make_tuple_indirect() able to handle plain attributes

The function has been introduced in 368202501539 to test at a low level
the new kinds of external toast datums, and would fail on OOM when
dealing with a plain storage attribute.  The existing tests of
indirect_toast do not test this case, still the error generated was
confusing.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/250a21e5-d677-6b2a-2692-cd4233785e37@gmail.com