decimal_round failed to perform a correct rounding 
of a decimal number if its first nine digits were '9'.
It just sets those digits to 0.


mysql-test/r/type_newdecimal.result:
  Bug #27984 Long Decimal Maths produces truncated results.
  test result
mysql-test/t/type_newdecimal.test:
  Bug #27984 Long Decimal Maths produces truncated results.
  test case
strings/decimal.c:
  Bug #27984 Long Decimal Maths produces truncated results.
  when to == from we break the data if we do to->buf[0]=0
  So now doing this after the data is moved and only
  if we really need to set to->buf[0] to zero