Fix for bug#42009: SELECT into variable gives different results to direct SELECT

Problem: storing "SELECT ... INTO @var ..." results in variables we used val_xxx()
methods which returned results of the current row. 
So, in some cases (e.g. SELECT DISTINCT, GROUP BY or HAVING) we got data
from the first row of a new group (where we evaluate a clause) instead of
data from the last row of the previous group.

Fix: use val_xxx_result() counterparts to get proper results.

sql/sql_class.cc

@@ -2052,7 +2052,7 @@ bool select_dumpvar::send_data(List<Item> &items)
     {
       Item_func_set_user_var *suv= new Item_func_set_user_var(mv->s, item);
       suv->fix_fields(thd, 0);
-      suv->check(0);
+      suv->save_item_result(item);
       suv->update();
     }
   }