message: Add zerocoinspend rules to IsStandard check.
diff: @@ -135,10 +135,19 @@ bool IsStandardTx(const CTransaction& tx, std::string& reason)
  return false;
  }
  - for (const CTxIn& txin : tx.vin)
  + bool fZerocoinSpend = false;
  + for (unsigned int i = 0; i < tx.vin.size(); i++)
  {
  - if (txin.scriptSig.IsZerocoinSpend())
  + const CTxIn& txin = tx.vin[i];
  + if (txin.scriptSig.IsZerocoinSpend()) {
  + if (!fZerocoinSpend && i > 0) {
  + reason = "malformed-zerocoinspend";
  + return false;
  + }
  + fZerocoinSpend = true;
  continue;
  + }
  +
  // Biggest 'standard' txin is a 15-of-15 P2SH multisig with compressed
  // keys (remember the 520 byte limit on redeemScript size). That works
  