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/[email protected]
Backpatch-through: 13

0 files changed, 0 insertions, 0 deletions