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_num_731(unsigned char *p, uint32_t value)
{
	archive_le32enc(p, value);
}

set_num_731(unsigned char *p, uint32_t value)
{
	archive_le32enc(p, value);
}
