Fix #114223: Crash when undo in curves edit mode

Regression in [0], missed scene assignment.

[0]: 585f43064a

source/blender/editors/curves/intern/curves_undo.cc

@@ -52,11 +52,12 @@ static bool step_encode(bContext *C, Main *bmain, UndoStep *us_p)
 {
   CurvesUndoStep *us = reinterpret_cast<CurvesUndoStep *>(us_p);
 
-  const Scene *scene = CTX_data_scene(C);
+  Scene *scene = CTX_data_scene(C);
   ViewLayer *view_layer = CTX_data_view_layer(C);
   uint objects_num = 0;
   Object **objects = ED_undo_editmode_objects_from_view_layer(scene, view_layer, &objects_num);
 
+  us->scene_ref.ptr = scene;
   new (&us->objects) Array<StepObject>(objects_num);
 
   threading::parallel_for(us->objects.index_range(), 8, [&](const IndexRange range) {