Analysis:
The problem lies in filesort.cc:find_all_keys().

When find_all_keys() is called for the outer query, it resets all
of the tree sets of fields - [read,write,vcol]_set and recomputes
them with respect to sorting.

However, in the loop for each current record the procedure calls
select->skip_record(thd), which evaluates the where clause, which
in turns evaluates the subquery. The JOIN evaluation of the
subquery eventually calls Field_long::val_int to evaluate the field
alias1.f1. The assertion condition
  "bitmap_is_set(table->read_set, field_index)"
fails, because the outer query changed the read_set of table "alias1".

Solution:
Restore the original read_set of the table before calling
SQL_SELECT::skip_record, then revert back to the read_set used in
find_all_keys.