Buffer over-run on all platforms, crash on windows, wrong result on other platforms,
when rounding numbers which start with 999999999 and have
precision = 9 or 18 or 27 or 36 ...


mysql-test/r/type_newdecimal.result:
  New test cases.
mysql-test/t/type_newdecimal.test:
  New test cases.
sql/my_decimal.h:
  Add sanity checking code, to catch buffer over/under-run.
strings/decimal.c:
  The original initialization of intg1 (add 1 if buf[0] == DIG_MAX)
  will set p1 to point outside the buffer, and the loop to copy the original value
      while (buf0 < p0)
        *(--p1) = *(--p0);
  will overwrite memory outside the my_decimal object.