MDEV-35793: Server crashes in Item_func_vec_distance_common::get_const_arg

The problem was caused by this scenario: The query had both SELECT DISTINCT
and ORDER BY. DISTINCT was converted into GROUP BY. Then, vector index was
used to resolve the GROUP BY.
When join_read_first() initialized vector index scan, it used the ORDER BY
clause instead of GROUP BY, which caused a crash.

Fixed by making test_if_skip_sort_order() remember which ordering the scan
produces in JOIN_TAB::full_index_scan_order, and join_read_first() using that.

sql/sql_select.h

@@ -556,6 +556,13 @@ typedef struct st_join_table {
   /** HAVING condition for checking prior saving a record into tmp table*/
   Item *having;
 
+  /**
+    Ordering to be produced when doing full index scan.
+    Important for vector indexes, set by test_if_skip_sort_order() when it
+    decides to use full index to produce rows in order.
+  */
+  ORDER *full_index_scan_order;
+
   /** TRUE <=> remove duplicates on this table. */
   bool distinct;