BufBlockstore copy_rec codec checks different from lotus

[magik6k]

• Lotus -> https://github.com/filecoin-project/lotus/blob/master/chain/vm/vm.go#L787-L796
• FVM -> https://github.com/filecoin-project/ref-fvm/blob/master/fvm/src/blockstore/buffered.rs#L192

[Stebalien]

@magik6k can you remember the actual issue here? I believe I've refactored this code since you reported the issue.