my_decimal in some cases can contain more decimal digits than
is officially supported (DECIMAL_MAX_PRECISION), so we need to
prepare bigger buffer for the resulting string.


mysql-test/r/type_newdecimal.result:
  bug #28361 Buffer overflow in DECIMAL code on Windows
  test result
mysql-test/t/type_newdecimal.test:
  bug #28361 Buffer overflow in DECIMAL code on Windows
  test case
  This test case doesn't fall in most cases even without the fix
  Still valgrind shows the problemn
sql/my_decimal.h:
  bug #28361 Buffer overflow in DECIMAL code on Windows
  DECIMAL_MAX_POSSIBLE_PRECISION introduced to be used in places,
  when we need to check for the number of digits technicaly possible
  in my_decimal.
  DECIMAL_MAX_STR_LENGTH fixed as it has to fit for the MAX_POSSIBLE_PRECISION