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.

sql/sql_select.h

@@ -178,6 +178,12 @@ typedef struct st_table_ref
   */
   bool          disable_cache;
 
+  /*
+    If true, this ref access was constructed from equalities generated by
+    LATERAL DERIVED (aka GROUP BY splitting) optimization
+  */
+  bool          uses_splitting;
+
   bool tmp_table_index_lookup_init(THD *thd, KEY *tmp_key, Item_iterator &it,
                                    bool value, uint skip= 0);
   bool is_access_triggered();