In the case that redistribution to fewer or one process happens on the coarsest
level, we failed to check whether the process really needs to compute something
here and did set up the smoother on all processes that had parts of the not yet
redistributed matrix on it. If the smoother did try to do communication with
the provided OwnerOverlapCopyCommunication then this rose an MPIError about an
invalid communicator.

With this commit we explicitly check whether the communicator has nonzero size
and only set up the smoother in this case.

Please note that all offered preconditioners in dune-istl do not do communication
in the constructor (yet).