ompi/datatype: Fix args of DARRAY

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.

ompi/datatype/ompi_datatype_args.c

@@ -764,7 +764,7 @@ static ompi_datatype_t* __ompi_datatype_create_from_args( int32_t* i, MPI_Aint*
         {
             const int* a_i[8] = {&i[0], &i[1], &i[2], &i[3 + 0 * i[0]], &i[3 + 1 * i[0]], &i[3 + 2 * i[0]],
                                  &i[3 + 3 * i[0]], &i[3 + 4 * i[0]]};
-            ompi_datatype_set_args( datatype, 4 * i[0] + 4,a_i, 0, NULL, 1, d, MPI_COMBINER_DARRAY);
+            ompi_datatype_set_args( datatype, 4 * i[2] + 4, a_i, 0, NULL, 1, d, MPI_COMBINER_DARRAY);
         }
         break;
         /******************************************************************/