Hangs with `DistArray.var` and `std`.

Code to reproduce, with 4 engines:

``` python
from distarray.dist import Context, Distribution
context = Context()
db = Distribution.from_shape(context, (14,), dist=('c',))
darr = context.ones(db)
darr.var()

# or 
#darr.std()
```

This also works for `'b'` distribution.
