Fix Multihash.decode <all zeros>

[singpolyma]

Previously a bytestring of all zeros would result in an exception being raised about the length being wrong, even with a correct length.

[kvdenden]

:+1:

I encountered the same issue when trying to decode this IPFS hash QmSdDyfTcVXHsqawjWBirhTU2rpPiDEVvPXgxfGeUD7Tqh

multihash_binary_string = Base58.base58_to_binary(ipfs_hash, :bitcoin)
multihash = Multihashes.decode(multihash_binary_string)

The digest has a trailing zero which causes Multihashes.decode to raise a Multihashes::DigestLengthError exception.

[kyledrake]

Thank you! (tests pass, Travis had an issue).