ORDER BY and LIMIT 1. 
The bug was introduced by the patch for bug 21727. The patch
erroneously skipped initialization of the array of headers
for sorted records for non-first evaluations of the subquery.

To fix the problem a new parameter has been added to the
function make_char_array that performs the initialization.
Now this function is called for any invocation of the 
filesort procedure. Yet it allocates the buffer for sorted
records only if this parameter is NULL.


mysql-test/r/subselect.result:
  Added a test case for bug #28811.
mysql-test/t/subselect.test:
  Added a test case for bug #28811.