coll/libnbc: fix MPI_Ireduce_scatter for single task communicator

when MPI_IN_PLACE is not used.

that commit fixes a regression introduced
open-mpi/ompi@49e79a9ade6c97e357391923f411e75e38fac346

(cherry picked from commit open-mpi/ompi@b9349d2eb9117c61205c98d5d2d5175d26971d23)