Fix T70441: crash after deleting a material output node

Caused by 5c79f2d0fb.
If no valid node_start is provided, we can just skip (e.g.
'ntree_shader_bump_branches' is not done then, but this is not
neccessary without a valid eevee output node anyways...).

Maniphest Tasks: T70441

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D5969

source/blender/blenkernel/intern/node.c

@@ -990,6 +990,10 @@ void nodeChainIterBackwards(const bNodeTree *ntree,
                             bool (*callback)(bNode *, bNode *, void *),
                             void *userdata)
 {
+  if (!node_start) {
+    return;
+  }
+
   /* Reset flag. */
   LISTBASE_FOREACH (bNode *, node, &ntree->nodes) {
     node->iter_flag = 0;