When switching to indexed ORDER BY we must be sure to reset the index read
flag if we are switching from a covering index to non-covering.

mysql-test/r/subselect.result:
  Bug#37548: test case
mysql-test/t/subselect.test:
  Bug#37548: test case
sql/sql_select.cc:
  Bug#37548: update the index read flag if the index for indexed ORDER BY is not
      covering.