cart_sub.c: allow remain_dims==NULL if there is no topology on comm

Patch submitted by Gilles Gouaillardet on #4518.  Reviewed by Jeff.

Fixes trac:4518

cmr=v1.8.2:reviewer=ompi-rm1.8

This commit was SVN r31517.

The following Trac tickets were found above:
  Ticket 4518 --> https://svn.open-mpi.org/trac/ompi/ticket/4518

ompi/mpi/c/cart_sub.c

@@ -13,6 +13,8 @@
  * Copyright (c) 2007-2009 Cisco Systems, Inc.  All rights reserved.
  * Copyright (c) 2012-2013 Los Alamos National Security, LLC.  All rights
  *                         reserved. 
+ * Copyright (c) 2014      Research Organization for Information Science
+ *                         and Technology (RIST). All rights reserved.
  * $COPYRIGHT$
  * 
  * Additional copyrights may follow
@@ -59,7 +61,8 @@ int MPI_Cart_sub(MPI_Comm comm, const int remain_dims[], MPI_Comm *new_comm)
             return OMPI_ERRHANDLER_INVOKE (comm, MPI_ERR_COMM,
                                           FUNC_NAME);
         }
-        if (NULL == remain_dims || NULL == new_comm) {
+        if (((NULL == remain_dims) && (0 != comm->c_topo->mtc.cart->ndims))
+            && (NULL == new_comm)) {
             return OMPI_ERRHANDLER_INVOKE (comm, MPI_ERR_ARG,
                                           FUNC_NAME);
         }