avoid 0 byte malloc.

This commit was SVN r17653.

ompi/mca/coll/sm2/coll_sm2_service.c

@@ -139,11 +139,15 @@ int setup_multinomial_tree(int tree_order, int num_nodes,
                         }
                     }
                     tree_nodes[node_index].n_children=n_children;
+                    if( 0 < n_children ) {
                         tree_nodes[node_index].children_ranks=(int *)
                             malloc(sizeof(int)*n_children);
                         if( NULL == tree_nodes[node_index].children_ranks) {
                             goto Error;
                         }
+                    } else {
+                        tree_nodes[node_index].children_ranks=NULL;
+                    }
                     /* fill in list */
                     for( rank=0 ; rank < n_children ; rank++ ) {
                         tree_nodes[node_index].children_ranks[rank]=