pgsql: Fix assertion failure in pg_prewarm() on objects without storage

Fix assertion failure in pg_prewarm() on objects without storage.

An assertion test added in commit 049ef33 could fail when pg_prewarm()
was called on objects without storage, such as partitioned tables.
This resulted in the following failure in assert-enabled builds:

Failed Assert("RelFileNumberIsValid(rlocator.relNumber)")

Note that, in non-assert builds, pg_prewarm() just failed with an error
in that case, so there was no ill effect in practice.

This commit fixes the issue by having pg_prewarm() raise an error early
if the specified object has no storage. This approach is similar to
the fix in commit 4623d7144 for pg_freespacemap.

Back-patched to v17, where the issue was introduced.

Author: Masahiro Ikeda <ikedamsh(at)oss(dot)nttdata(dot)com>
Reviewed-by: Dilip Kumar <dilipbalaut(at)gmail(dot)com>
Reviewed-by: Richard Guo <guofenglinux(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/e082e6027610fd0a4091ae6d033aa117@oss.nttdata.com
Backpatch-through: 17

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b64c585fde77206ee54747f31035dde56eee6bf9

Modified Files
--------------
contrib/pg_prewarm/Makefile | 2 ++
contrib/pg_prewarm/expected/pg_prewarm.out | 10 ++++++++++
contrib/pg_prewarm/meson.build | 5 +++++
contrib/pg_prewarm/pg_prewarm.c | 8 ++++++++
contrib/pg_prewarm/sql/pg_prewarm.sql | 10 ++++++++++
5 files changed, 35 insertions(+)