Make it work with appveyor's outdated OSG build

components/resource/objectcache.cpp

@@ -129,10 +129,13 @@ void ObjectCache::accept(osg::NodeVisitor &nv)
         ++itr)
     {
         osg::Object* object = itr->second.first.get();
-        osg::Node* node = object->asNode();
+        if (object)
+        {
+            osg::Node* node = dynamic_cast<osg::Node*>(object);
             if (node)
                 node->accept(nv);
         }
     }
+}
 
 }