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

src/pl/plpgsql/src/expected/plpgsql_record.out

@@ -476,6 +476,12 @@ select sillyaddtwo(42);
           44
 (1 row)
 
+select sillyaddtwo(43);
+ sillyaddtwo 
+-------------
+          45
+(1 row)
+
 -- check access to system columns in a record variable
 create function sillytrig() returns trigger language plpgsql as
 $$begin

src/pl/plpgsql/src/pl_exec.c

@@ -6858,7 +6858,16 @@ revalidate_rectypeid(PLpgSQL_rec *rec)
 	Assert(typ != NULL);
 	if (typ->tcache &&
 		typ->tcache->tupDesc_identifier == typ->tupdesc_id)
-		return;					/* known up-to-date */
+	{
+		/*
+		 * Although *typ is known up-to-date, it's possible that rectypeid
+		 * isn't, because *rec is cloned during each function startup from a
+		 * copy that we don't have a good way to update.  Hence, forcibly fix
+		 * rectypeid before returning.
+		 */
+		rec->rectypeid = typ->typoid;
+		return;
+	}
 
 	/*
 	 * typcache entry has suffered invalidation, so re-look-up the type name

src/pl/plpgsql/src/sql/plpgsql_record.sql

@@ -303,6 +303,7 @@ drop table mutable2;
 select sillyaddtwo(42);  -- fail
 create table mutable2(f0 text, f1 int, f2 text);
 select sillyaddtwo(42);
+select sillyaddtwo(43);
 
 -- check access to system columns in a record variable