fix implementation of yubikey secret reading

Improve error reporting and fix crash

src/keepass2android-app/KeeChallenge.cs

@@ -148,17 +148,13 @@ namespace KeeChallenge
                 {
                     //read the secret from the stream
                     int totalBytesRead = 0;
-                    byte[] buffer = new byte[secret.Length];
-                    var secretOutputStream = new MemoryStream(secret);
 
-                    int bytesRead = csDecrypt.Read(buffer, totalBytesRead, secret.Length - totalBytesRead);
+                    int bytesRead = csDecrypt.Read(secret, totalBytesRead, secret.Length - totalBytesRead);
                     while (bytesRead > 0 && totalBytesRead < secret.Length)
                     {
-                        secretOutputStream.Write(buffer, 0, bytesRead);
                         totalBytesRead += bytesRead;
-                        bytesRead = csDecrypt.Read(buffer, totalBytesRead, secret.Length - totalBytesRead);
+                        bytesRead = csDecrypt.Read(secret, totalBytesRead, secret.Length - totalBytesRead);
                     }
-                    secretOutputStream.Close();
 
                     csDecrypt.Close();
                 }