Build fails with `Py_STACKREF_DEBUG` set.

# Bug report

### Bug description:

I think this is a relatively recent regression, probably in the last month or so.

Can be reproduced by uncommenting the line:
```
// #define Py_STACKREF_DEBUG 1
```
in pycore_structs.h


### CPython versions tested on:

CPython main branch

### Operating systems tested on:

Linux