Problem: make_sortorder() asserts in debug mode: if we are
ordering by an Item_ref, it must be an Item_direct_view_ref

In 5.7, explain for the offending query (see testcase) is
id      select_type        table   ... Extra
1       PRIMARY            t       ... Using temporary; Using filesort
2       DEPENDENT SUBQUERY g       ... Using index; Using filesort
So we are sorting table 'g' rather than a derived/temporary table.
And 'a' is a reference to the outer table.

Solution: Relax the debug assert to allow 'Item_ref' objects.