Inplace op with binned var on RHS does a reduction operation

```python
import scipp as sc

da = sc.data.table_xyz(1000).bin(x=4)
x = da.bins.coords['x']
var = sc.arange('x', 4, dtype='float')
var *= x  # This should raise `DimensionError`
```
`var` has its values overwritten with uninitialized values. This in-place op should raise!