Fix T81250: Crash after undoing with prefetching

Main DB and it's structs can point to different address after undoing.

In this case problem was that bmain was not updated. Same fix was
done for scene as well.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9240

source/blender/sequencer/intern/prefetch.c

@@ -319,6 +319,7 @@ static void seq_prefetch_update_scene(Scene *scene)
     return;
   }
 
+  pfjob->scene = scene;
   seq_prefetch_free_depsgraph(pfjob);
   seq_prefetch_init_depsgraph(pfjob);
 }
@@ -499,15 +500,14 @@ static PrefetchJob *seq_prefetch_start(const SeqRenderData *context, float cfra)
       BLI_mutex_init(&pfjob->prefetch_suspend_mutex);
       BLI_condition_init(&pfjob->prefetch_suspend_cond);
 
-      pfjob->bmain = context->bmain;
       pfjob->bmain_eval = BKE_main_new();
-
       pfjob->scene = context->scene;
       seq_prefetch_init_depsgraph(pfjob);
     }
   }
   seq_prefetch_update_scene(context->scene);
   seq_prefetch_update_context(context);
+  pfjob->bmain = context->bmain;
 
   pfjob->cfra = cfra;
   pfjob->num_frames_prefetched = 1;