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.
isoent_cmp_key(const struct archive_rb_node *n, const void *key)
{
	const struct isoent *e = (const struct isoent *)n;

	return (strcmp(e->file->basename.s, (const char *)key));
}

isoent_cmp_key(const struct archive_rb_node *n, const void *key)
{
	const struct isoent *e = (const struct isoent *)n;

	return (strcmp(e->file->basename.s, (const char *)key));
}
