Caused by a difference in the size of the digest
byte array allocated on Haskell side and the
amount of bytes copied to it on the C side.
In cbits/cryptonite_sha512.c:cryptonite_sha384_finalize
SHA384_DIGEST_SIZE bytes is copied into the out buffer.
SHA384_DIGEST_SIZE is #defined as 64 in cbits/cryptonite_sha512.h
while the buffer given will have size 48, as defined in
Crypto/Hash/SHA384.hs.
Defining SHA384_DIGEST_SIZE as 48 fixes the issue.
The output size is now passed by parameter to the finalize function
instead of being stored in the context. that simplify quite a
bit the passing of this parameter
