The bug caused a crash of the server if a subquery with
ORDER BY DESC used the range access method.
The bug happened because the method QUICK_SELECT_DESC::reset
was not reworked after MRR interface had been introduced.


mysql-test/r/subselect.result:
  Added a test case for bug #20869.
mysql-test/t/subselect.test:
  Added a test case for bug #20869.