message: bitcoin-tx: Use constant for n pubkeys check
  Use the constant for the maximum number of public keys in a multisig
  script defined in script/script.h instead of hardcoding it.
diff: @@ -356,7 +356,7 @@ static void MutateTxAddOutMultiSig(CMutableTransaction& tx, const std::string& s
  if (vStrInputParts.size() < numkeys + 3)
  throw std::runtime_error("incorrect number of multisig pubkeys");
  - if (required < 1 || required > 20 || numkeys < 1 || numkeys > 20 || numkeys < required)
  + if (required < 1 || required > MAX_PUBKEYS_PER_MULTISIG || numkeys < 1 || numkeys > MAX_PUBKEYS_PER_MULTISIG || numkeys < required)
  throw std::runtime_error("multisig parameter mismatch. Required " \
  + std::to_string(required) + " of " + std::to_string(numkeys) + "signatures.");
  