hasher: fix error from trying to stop an already-stopped db

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Before this change, Hasher would sometimes try to stop a bolt db that was
already stopped, resulting in an error. This change fixes the issue by checking
first whether the db is already stopped.

https://forum.rclone.org/t/hasher-with-gdrive-backend-does-not-return-sha1-sha256-for-old-files/44680/11?u=nielash

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