avoid calling omp_dpm.mark_dyncomm if the size of the local communicator

is zero. The routine assumes that at least one process is available in the
group, which lead to a segfault when creating communicators with GROUP_EMPTY.

Fixes trac:2752

This commit was SVN r24595.

The following Trac tickets were found above:
  Ticket 2752 --> https://svn.open-mpi.org/trac/ompi/ticket/2752

ompi/communicator/comm.c

@@ -145,7 +145,9 @@ int ompi_comm_set ( ompi_communicator_t **ncomm,
     
     /* Check how many different jobids are represented in this communicator.
        Necessary for the disconnect of dynamic communicators. */
+    if ( local_size != 0 ) {
 	ompi_dpm.mark_dyncomm (newcomm);
+    }
 
     /* Set error handler */
     newcomm->error_handler = errh;