This was a buffer overrun in do_div_mod(), overwriting the internal buffer
of auto variable 'tmp' in Item_func_int_div::val_int.
Result on windows: 'this' is set to zero, and crash.
Ran fine on other platforms (no valgrind warnings),
but this is undefined behaviour on any platform of course.

include/decimal.h:
  Add const qualifiers to function prototypes which are used by sql/my_decimal.h
mysql-test/r/func_math.result:
  New test case.
mysql-test/t/func_math.test:
  New test case.
sql/my_decimal.h:
  Remove several C-style casts:
   - some of the were up-casts, and thus un-necessary
   - some of them should have been const-casts, but it is better to make the
     underlying library functions in (decimal.[h|c]) const instead.
strings/decimal.c:
  Check for buffer overrun in do_div_mod()
  Add const qualifiers to functions which are used by sql/my_decimal.h