Don't access event after it was deleted

- NeoChat stores pointer to event
- Event is replaced
- libQuotient deletes the event and notifies us that the event changes
- We're accessing the event to check its id
- Boom

Make this not boom by accessing the ID that we're additionally storing anyway.

src/models/messagecontentmodel.h

@@ -114,6 +114,7 @@ Q_SIGNALS:
 private:
     QPointer<NeoChatRoom> m_room;
     QString m_eventId;
+    QString m_eventSenderId;
     const Quotient::RoomEvent *m_event = nullptr;
 
     bool m_isPending;