Dispose cipher object in time

Author: scott-xu

src/Renci.SshNet/PrivateKeyFile.PuTTY.cs

@@ -116,8 +116,10 @@ public Key Parse()
                                 throw new SshException("PuTTY key file version " + _version + " is not supported");
                         }
 
-                        var cipher = new AesCipher(cipherKey, cipherIV, AesCipherMode.CBC, pkcs7Padding: false);
-                        privateKey = cipher.Decrypt(_data);
+                        using (var cipher = new AesCipher(cipherKey, cipherIV, AesCipherMode.CBC, pkcs7Padding: false))
+                        {
+                            privateKey = cipher.Decrypt(_data);
+                        }
 
                         break;
                     case "none":