unpack_iterator type has not been registered into _struct module. And all users get only unpack_iterator object from function returning iterator. The object iterating doesn't need reference to type. Therefore, I think issue finish by PyType_Ready fix because this segfault doesn't have big impact.
However, I think if possible, it is preferable that extension module having un-initialize type objects gets a compile error at build time. And impossible case for compile error is better that wrong extension module always fails import.