Prevent exponential growth of length of edges in clusters

`edgelabel_ranks` gets called for the root graph and all of its cluster
subgraphs. It should only double `ED_minlen` for each edge once. Reverting the
test for labeled edges to check only the active graph, rather than its root,
prevents it from expanding the `minlen` once for every level of cluster
containing the edge.

Fixes ellson#1246

lib/dotgen/rank.c

@@ -96,7 +96,7 @@ edgelabel_ranks(graph_t * g)
     node_t *n;
     edge_t *e;
 
-    if (GD_has_labels(g->root) & EDGE_LABEL) {
+    if (GD_has_labels(g) & EDGE_LABEL) {
 	for (n = agfstnode(g); n; n = agnxtnode(g, n))
 	    for (e = agfstout(g, n); e; e = agnxtout(g, e))
 		ED_minlen(e) *= 2;