return 0 instead of crash on agnxtsubg(g) when g is a root graph

diff --git a/lib/cgraph/subg.c b/lib/cgraph/subg.c
index 3f9c84de1374542688a642a8f58b14235431c3a7..da4ceb17dc92300855289a97d60e3fd894b26449 100644 (file)
--- a/lib/cgraph/subg.c
+++ b/lib/cgraph/subg.c
@@ -76,7 +76,7 @@ Agraph_t *agnxtsubg(Agraph_t * subg)
     Agraph_t *g;
 
     g = agparent(subg);
-    return (Agraph_t *) dtnext(g->g_dict, subg);
+    return g? (Agraph_t *) dtnext(g->g_dict, subg) : 0;
 }
 
 Agraph_t *agparent(Agraph_t * g)