message: Slightly refactor ProcessInstantSendLock
  Rewrite `if (var =  func())` in a less confusing way
diff: @@ -659,12 +659,14 @@ void CInstantSendManager::ProcessInstantSendLock(NodeId from, const uint256& has
  if (db.GetInstantSendLockByHash(hash)) {
  return;
  }
  - if (otherIsLock = db.GetInstantSendLockByTxid(islock.txid)) {
  + otherIsLock = db.GetInstantSendLockByTxid(islock.txid);
  + if (otherIsLock != nullptr) {
  LogPrint("instantsend", "CInstantSendManager::%s -- txid=%s, islock=%s: duplicate islock, other islock=%s, peer=%d\n", __func__,
  islock.txid.ToString(), hash.ToString(), ::SerializeHash(*otherIsLock).ToString(), from);
  }
  for (auto& in : islock.inputs) {
  - if (otherIsLock = db.GetInstantSendLockByInput(in)) {
  + otherIsLock = db.GetInstantSendLockByInput(in);
  + if (otherIsLock != nullptr) {
  LogPrint("instantsend", "CInstantSendManager::%s -- txid=%s, islock=%s: conflicting input in islock. input=%s, other islock=%s, peer=%d\n", __func__,
  islock.txid.ToString(), hash.ToString(), in.ToStringShort(), ::SerializeHash(*otherIsLock).ToString(), from);
  }
  