Item_substr's results are improperly stored in a temporary table due to       
wrongly calculated max_length value for multi-byte charsets if two            
arguments specified.                                                          


mysql-test/r/ctype_utf8.result:
  Fix for bug #20204: "order by" changes the results returned
    - test case.
mysql-test/t/ctype_utf8.test:
  Fix for bug #20204: "order by" changes the results returned
    - test result.
sql/item_strfunc.cc:
  Fix for bug #20204: "order by" changes the results returned
    - always take into account current mbmaxlen value calculating max_length.