Added a test case for bug #6866.
sql_select.cc:
  Fixed bug #6866.
  Bug was due to the fact that on_expr was not backed up
  for the second execution of the stored procedure.


sql/sql_select.cc:
  Fixed bug #6866.
  Bug was due to the fact that on_expr was not backed up
  for the second execution of the stored procedure.
mysql-test/r/sp.result:
  Added a test case for bug #6866.
mysql-test/t/sp.test:
  Added a test case for bug #6866.