from mysql-next-mr-opt-backporting.

Bug#54515: Crash in opt_range.cc::get_best_group_min_max on 
           SELECT from VIEW with GROUP BY
      
When handling the grouping items in get_best_group_min_max, the
items need to be of type Item_field. In this bug, an ASSERT 
triggered because the item used for grouping was an 
Item_direct_view_ref (i.e., the group column is from a view). 
The fix is to get the real_item since Item_ref* pointing to 
Item_field is ok.