Fix potential assertion failure/crash

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@10694 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/graphics/lod_node.cpp

@@ -111,6 +111,8 @@ void LODNode::OnRegisterSceneNode()
             {
                 scene::IMeshBuffer* mb = mesh->getMeshBuffer(n);
                 video::ITexture* t = mb->getMaterial().getTexture(0);
+                if (t == NULL) continue;
+                
                 Material* m = material_manager->getMaterialFor(t, mb);
                 if (m != NULL)
                 {