message: Don't pass the genesis block through CheckWork
  The genesis block has no prevhash, and no pindexPrev (which CheckWork
  uses heavily), and doesn't require it's work being checked.
  fixes
diff: @@ -3559,7 +3559,7 @@ bool AcceptBlock(CBlock& block, CValidationState& state, CBlockIndex** ppindex,
  return state.DoS(100, error("%s : prev block invalid", __func__), REJECT_INVALID, "bad-prevblk");
  }
  - if (!CheckWork(block, pindexPrev))
  + if (block.GetHash() != Params().HashGenesisBlock() && !CheckWork(block, pindexPrev))
  return false;
  if (!AcceptBlockHeader(block, state, &pindex))
  