It was actually OK in def functions. It only looks very dodgy:
```
__Pyx_XDEC_MEMVIEW(closure->arg)
```
This gets called twice and `INC` gets called once. However this is
actually OK since XDEC really means "clear"

Fixes https://github.com/cython/cython/issues/4798
for 0.29.x (completely I think)