Issue 711:  Be more careful about verifying filename lengths when writing ISO9660 archives

* Don't cast size_t to int, since this can lead to overflow
  on machines where sizeof(int) < sizeof(size_t)
* Check a + b > limit by writing it as
    a > limit || b > limit || a + b > limit
  to avoid problems when a + b wraps around.
set_digit(unsigned char *p, size_t s, int value)
{

	while (s--) {
		p[s] = '0' + (value % 10);
		value /= 10;
	}
}

set_digit(unsigned char *p, size_t s, int value)
{

	while (s--) {
		p[s] = '0' + (value % 10);
		value /= 10;
	}
}
