treestore: Fix segfault when reordering non-existing children

Calling gtk_tree_store_reorder() on a node without children would cause a
segfault; Fixes https://bugzilla.gnome.org/show_bug.cgi?id=698396

Signed-off-by: Olivier Brunel <jjk@jjacky.com>

gtk/gtktreestore.c

@@ -2280,6 +2280,12 @@ gtk_tree_store_reorder (GtkTreeStore *tree_store,
   else
     level = G_NODE (parent->user_data)->children;
 
+  if (G_UNLIKELY (!level))
+    {
+      g_warning ("%s: Cannot reorder, parent has no children", G_STRLOC);
+      return;
+    }
+
   /* count nodes */
   node = level;
   while (node)