fixed my_charpos() call in Field_varstring::get_key_image


mysql-test/t/type_varchar.test:
  BUG#9339 - Updating a column does not work if the table has a UTF-8 VARCHAR primary key
mysql-test/r/type_varchar.result:
  BUG#9339 - Updating a column does not work if the table has a UTF-8 VARCHAR primary key