Correctly validate hashReserved

[str4d]

The hashReserved field in the Zcash block header is currently all-zero by convention, but will likely be used in future, and thus should not be assumed to be all-zero. It should instead be validated as an unknown 32-byte value, in the same way as hashPrevBlock and hashMerkleRoot, and then used as-provided by the Stratum server.