Fix random BFS traversal failures (#621)

Essentially a redo of PR #612 which is erased by git merge somehow.

tests/compute/test_traversal.py

@@ -31,8 +31,10 @@ def test_bfs(n=100):
                 edges_nx.append(edge_frontier)
                 frontier = set([v])
                 edge_frontier = set([g.edge_id(u, v)])
-        layers_nx.append(frontier)
-        edges_nx.append(edge_frontier)
+        # avoids empty successors
+        if len(frontier) > 0 and len(edge_frontier) > 0:
+            layers_nx.append(frontier)
+            edges_nx.append(edge_frontier)
         return layers_nx, edges_nx
 
     g = dgl.DGLGraph()