MDEV-30240 Wrong result upon aggregate function with SQL_BUFFER_RESULT

The problem was that when storing rows into a temporary table,
MIN/MAX items that where marked as constants (as theire value had
been computed at start of query) would be reset.

Fixed by not reseting MIN/MAX items that are marked as const in
Item_sum_min_max::clear().

sql/sql_select.cc

@@ -25871,7 +25871,6 @@ bool JOIN::alloc_func_list()
   @param field_list        All items
   @param send_result_set_metadata       Items in select list
   @param before_group_by   Set to 1 if this is called before GROUP BY handling
-  @param recompute         Set to TRUE if sum_funcs must be recomputed
 
   @retval
     0  ok