Fields of view represented by Item_direct_view_ref. When complex expression
such as if(sum()>...,...) is splited in simpler parts by refs was ignored.
Beside this direct ref doesn't use it's result_field and thus can't store
it's result in tmp table which is needed for sum() ... group.
All this results in reported bug.

Item::split_sum_func2() now converts Item_direct_view_ref to Item_ref to
make fields from view being storable in tmp table.