pg_buffercache: restore rowtype verification in pg_buffercache_pages()

pg_buffercache: restore rowtype verification in pg_buffercache_pages()

Commit 257c8231bf9 changed pg_buffercache_pages() to materialize its output
directly into a tuplestore. As a result, the function ended up trusting
a caller-supplied RECORD descriptors. That could lead to crashes
if the supplied row definition did not match the actual returned values,
for example by passing bool Datums to tuplestore_putvalues() with
an incompatible descriptor.

Fix this by constructing the correct tuple descriptor for
pg_buffercache_pages() and assigning it to
rsinfo->setDesc after InitMaterializedSRF(). This restores the executor's
tupledesc_match() verification, so incompatible caller-supplied
row definitions are rejected with an error, as before commit 257c8231bf9.

Bug: #19508
Reported-by: Nikita Kalinin <n.kalinin@postgrespro.ru>
Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Reviewed-by: Ashutosh Sharma <ashu.coek88@gmail.com>
Discussion: https://postgr.es/m/19508-e5f188183279219b@postgresql.org