OmemoManager: Remove private fallback markers before serializing

This is needed to avoid that private markers leak into the public parts
of the message.

src/omemo/QXmppOmemoManager_p.cpp

@@ -992,6 +992,11 @@ QXmppTask<QXmppE2eeExtension::MessageEncryptResult> ManagerPrivate::encryptMessa
  // Whether to advise the server to store other kinds of messages is up to the
  // client.
  // That facilitates a consistent handling of message processing hints.
+
+ // reset fallback markers: they are serialized in both public and private modes,
+ // so this is needed to avoid leaking sensitive content
+ message.setFallbackMarkers({});
+
  if (!message.body().isEmpty() || message.trustMessageElement()) {
  QXmppFallback fallback { ns_omemo_2.toString(), { { QXmppFallback::Body, {} } } };