waylandserver: fix race condition

When the xwayland connection gets destroyed, it doesn't get deleted immediately. If the
xwayland scale changes in between WaylandServer::destroyXWaylandConnection and the actual
destruction of the connection, KWin would crash

src/wayland_server.cpp

@@ -693,7 +693,9 @@ int WaylandServer::createXWaylandConnection()
 
     m_xwaylandConnection->setScaleOverride(kwinApp()->xwaylandScale());
     connect(kwinApp(), &Application::xwaylandScaleChanged, m_xwaylandConnection, [this]() {
-        m_xwaylandConnection->setScaleOverride(kwinApp()->xwaylandScale());
+        if (m_xwaylandConnection) {
+            m_xwaylandConnection->setScaleOverride(kwinApp()->xwaylandScale());
+        }
     });
 
     return socket.fd;