According to MPI-3.1 P.122, `ni` for `MPI_COMBINER_DARRAY`
should be `4*ndims+4`, not `4*size+4`.
This bug may cause SEGV if `size` is smaller than `ndims`
when the darray is used for one-sided communication (pt2pt OSC).
This bug was introduced in open-mpi/ompi@79b13f36 (when darray
became a first class citizen and the `a_i` index of darray was
shifted by 2). The corresponding `MPI_Type_create_darray()`
function sets a right value so we don't need to update the function.
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