Fixed bug that let clients log into the anonymous user without providing a username

Before, the server accepted an empty username as anonymous username, even though it reported an error.
Now, the server still reports an error, but also rejects the login. Clients have to use "anonymous" or "ftp" as username to log in as anonymous user and cannot leave the username empty anymore.

Author: FlorianReimold

fineftp-server/src/ftp_session.cpp

@@ -321,7 +321,7 @@ namespace fineftp
 
   void FtpSession::handleFtpCommandPASS(const std::string& param)
   {
-    if (last_command_ != "USER")
+    if (last_command_ != "USER" || username_for_login_.empty())
     {
       sendFtpMessage(FtpReplyCode::COMMANDS_BAD_SEQUENCE, "Please specify username first");
       return;