message: Handle empty sender identity in MslControl.receive()
diff: @@ -1212,7 +1212,7 @@ shared_ptr<MessageInputStream> MslControl::receive(shared_ptr<MslContext> ctx,
  // or if the remote entity identity is equal to this entity
  // identity.
  const string sender = (masterToken) ? responseHeader->getSender() : entityAuthData->getIdentity();
  - if ((masterToken && masterToken->isDecrypted() && masterToken->getIdentity() != sender))
  + if ((masterToken && masterToken->isDecrypted() && !sender.empty() && masterToken->getIdentity() != sender))
  {
  stringstream ss;
  ss << "sender " << sender << "; master token " << masterToken->getIdentity();
  