Prevent assertion failure in contrib/pg_freespacemap.

Prevent assertion failure in contrib/pg_freespacemap.

Applying pg_freespacemap() to a relation lacking storage (such as a
view) caused an assertion failure, although there was no ill effect
in non-assert builds.  Add an error check for that case.

Bug: #18866
Reported-by: Robins Tharakan <[email protected]>
Author: Tender Wang <[email protected]>
Reviewed-by: Euler Taveira <[email protected]>
Discussion: https://postgr.es/m/18866-d68926d0f1c72d44@postgresql.org
Backpatch-through: 13