mpi_redux: fix division by zero

mpi/src/starpu_mpi_task_insert.c

@@ -806,6 +806,10 @@ void starpu_mpi_redux_data_prio_tree(MPI_Comm comm, starpu_data_handle_t data_ha
 		_STARPU_MPI_DEBUG(5, "mpi_data->redux_map[%d] = %d\n", i, mpi_data->redux_map[i]);
 		if (mpi_data->redux_map[i]) nb_contrib++;
 	}
+	if (nb_contrib == 0) {
+		/* Nothing to do! */
+		return;
+	}
 	if (arity < 2)
 	{
 		arity = nb_contrib;