Ensure that the hashReserved field is correctly parsed

[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.

Obviously I can't check the source code to tell whether or not you already do this 😄 and I don't have the time or graphics cards to experiment with the miner binary, so I'm opening this issue to ask that you check this.