MDEV-23723: Crash when test_if_skip_sort_order() is checked for derived ...

The problem was caused by the following scenario:

Subquery's table has two indexes, KEY a(a), KEY a_b(a,b)

- LATERAL DERIVED optimization decides to use index a.
  = The subquery uses ref access over key a.
- test_if_skip_sort_order() sees that KEY a_b satisfies the
  subquery's GROUP BY clause, and attempts to switch to it.
  = It fails to do so, because KEYUSE objects for index a_b
    are switched off.

Fixed by disallowing to change the ref access key if it uses KEYUSE
objects injected by LATERAL DERIVED optimization.

mysql-test/main/derived_split_innodb.test

@@ -124,3 +124,29 @@ eval set statement optimizer_switch='split_materialized=off' for explain $q;
 drop view v1;
 
 drop table t1,t2;
+
+--echo #
+--echo #  MDEV-23723: Crash when test_if_skip_sort_order() is checked for derived table subject to split
+--echo #
+CREATE TABLE t1 (a INT, b INT, KEY (a), KEY (a,b)) ENGINE=InnoDB;
+CREATE TABLE t2 (c INT, KEY (c)) ENGINE=InnoDB;
+
+SELECT * FROM t1 t1a JOIN t1 t1b;
+
+INSERT INTO t2 VALUES (1),(2);
+INSERT INTO t1 VALUES (1,2),(3,4),(5,6),(7,8),(9,10),(11,12);
+
+let $query=
+EXPLAIN
+SELECT *
+FROM
+  t1 JOIN
+  (SELECT t1.a, t1.b FROM t1, t2 WHERE t1.b = t2.c GROUP BY t1.a, t1.b) as dt
+WHERE
+  t1.a = dt.a;
+
+eval set statement optimizer_switch='split_materialized=off' for $query;
+eval set statement optimizer_switch='split_materialized=on' for  $query;
+
+DROP TABLE t1, t2;
+