Don't crash when leaving an opened room

Should fix #568

src/chatdocumenthandler.cpp

@@ -105,7 +105,7 @@ ChatDocumentHandler::ChatDocumentHandler(QObject *parent)
 {
     connect(this, &ChatDocumentHandler::roomChanged, this, [this]() {
         m_completionModel->setRoom(m_room);
-        static NeoChatRoom *previousRoom = nullptr;
+        static QPointer<NeoChatRoom> previousRoom = nullptr;
         if (previousRoom) {
             disconnect(previousRoom, &NeoChatRoom::chatBoxTextChanged, this, nullptr);
         }

src/models/userlistmodel.h

@@ -7,6 +7,7 @@
 
 #include <QAbstractListModel>
 #include <QObject>
+#include <QPointer>
 
 class NeoChatRoom;
 
@@ -70,7 +71,7 @@ private Q_SLOTS:
     void avatarChanged(Quotient::User *user, const Quotient::Room *context);
 
 private:
-    NeoChatRoom *m_currentRoom;
+    QPointer<NeoChatRoom> m_currentRoom;
     QList<Quotient::User *> m_users;
 
     int findUserPos(Quotient::User *user) const;