This associates the current batch with the boundary when entering pending mode. That way other async work associated to that boundary also can associate itself with that batch, even if e.g. due to flushing it is no longer the current batch.This solves a null pointer exception that can occur when the batch is flushed before the next top level await or async derived gets a hold of the current batch, which is null then.Fixes#16596Fixessveltejs/kit#14124