Fix CVE-2021-31855

Deleting an attachment of a decrypted encrypted message stored on a remote server
(e.g. an IMAP server) causes KMail to upload the decrypted content of the message
to the remote server. This is not easily noticeable by the user because KMail does
not display the decrypted content.

messageviewer/src/viewer/viewer_p.cpp

@@ -396,7 +396,7 @@ bool ViewerPrivate::deleteAttachment(KMime::Content *node, bool showWarning)
 
     KMime::Message *modifiedMessage = mNodeHelper->messageWithExtraContent(mMessage.data());
     mMimePartTree->mimePartModel()->setRoot(modifiedMessage);
-    mMessageItem.setPayloadFromData(modifiedMessage->encodedContent());
+    mMessageItem.setPayloadFromData(mMessage->encodedContent());
     auto job = new Akonadi::ItemModifyJob(mMessageItem, mSession);
     job->disableRevisionCheck();
     connect(job, &KJob::result, this, &ViewerPrivate::itemModifiedResult);