create skip leaks CacheEntry

```
    DEBUG_TRACE("create skip %s", name(frame));
    Py_DECREF(result);
    set_extra(frame->f_code, SKIP_CODE);
    // Re-enable custom behavior
    eval_frame_callback_set(callback);
    return eval_frame_default(tstate, frame, throw_flag);
```

set extra doesn't deallocate the previous extra entry of f code. If it was a cache entry, that means it will be leaked.