message: Don't accept standalone coinstake transactions
diff: @@ -615,6 +615,10 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
  if (tx.IsCoinBase())
  return state.DoS(100, false, REJECT_INVALID, "coinbase");
  + // Coinstake is only valid in a block, not as a loose transaction
  + if (tx.IsCoinStake())
  + return state.DoS(100, false, REJECT_INVALID, "coinstake");
  +
  // Rather not work on nonstandard transactions (unless -testnet/-regtest)
  std::string reason;
  if (fRequireStandard && !IsStandardTx(tx, reason))
  