pgsql: Fix possible loss of sync between rectypeid and underlying PLpgS

Fix possible loss of sync between rectypeid and underlying PLpgSQL_type.

When revalidate_rectypeid() acts to update a stale record type OID
in plpgsql's data structures, it fixes the active PLpgSQL_rec struct
as well as the PLpgSQL_type struct it references.  However, the latter
is shared across function executions while the former is not.  In a
later function execution, the PLpgSQL_rec struct would be reinitialized
by copy_plpgsql_datums and would then contain a stale type OID,
typically leading to "could not open relation with OID NNNN" errors.
revalidate_rectypeid() can easily fix this, fortunately, just by
treating typ->typoid as authoritative.

Per report and diagnosis from Ashutosh Sharma, though this is not his
suggested fix.  Back-patch to v11 where this code came in.

Discussion: https://postgr.es/m/CAE9k0Pkd4dZwt9J5pS9xhJFWpUtqs05C9xk_GEwPzYdV=GxwWg@mail.gmail.com

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ee206cb830eff45179afde06c93643210b52b196

Modified Files
--------------
src/pl/plpgsql/src/expected/plpgsql_record.out |  6 ++++++
src/pl/plpgsql/src/pl_exec.c                   | 11 ++++++++++-
src/pl/plpgsql/src/sql/plpgsql_record.sql      |  1 +
3 files changed, 17 insertions(+), 1 deletion(-)