libscrypt_mcf returns 0 on failure, not -1
Sanity check N > 1 in libscrypt_scrypt
Merge pull request #18 from jvarho/N-bounds-fix

Sanity check N > 1 in libscrypt_scrypt
Merge pull request #17 from jvarho/return-value-fix

Fix libscrypt_mcf return value check in libscrypt_hash
More explicit fix for #18 as per splint advice.
Following #16, it's now plausible for this function to fail.
Use existing define instead of magic number
Define and use SCRYPT_SALT_LEN to 16