Analysis:
Before calling:
  write_record= (select->skip_record(thd) > 0);
the function find_all_keys needs to restore the original read/write
sets of the table that is sorted if the condition select->cond
contains a subquery.

This didn't happen in this test case because the flag "with_subselect"
was not set properly for select->cond.

The reason for the flag not being set properly, was that this condition
was rewritten by add_cond_and_fix() inside make_join_select() by:

      /* Add conditions added by add_not_null_conds(). */
      if (tab->select_cond)
        add_cond_and_fix(thd, &tmp, tab->select_cond);

However, the function add_cond_and_fix() called the shortcut method
Item::quick_fix_field() that didn't update the "with_subselect"
property.

Solution:
Call the complete Item::fix_fields() to update all Item properties,
including "with_subselect".