calculate Item_func_format::max_length using charset->mbmaxlen


mysql-test/r/func_math.result:
  Fix for bug#16678 FORMAT gives wrong result if client run with default-character-set=utf8
    test case
mysql-test/t/func_math.test:
  Fix for bug#16678 FORMAT gives wrong result if client run with default-character-set=utf8
    test case