Fix -fcheck-bounds for compareBytesArray and setByteArray

As discussed in !7133 (comment 403462) and !7133 (comment 403468).

@maerwald hit both issues while testing bytestring with -fcheck-bounds: https://github.com/haskell/bytestring/pull/471#issuecomment-1019357810 and https://github.com/haskell/bytestring/issues/473

(Thanks for implementing this flag, it's tremendously useful)

Edited Jan 23, 2022 by Bodigrim

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