Reset mFrameCallback in destruction

Otherwise we crash if we call reset twice. For example if we call
setParent and setVisible together.

Change-Id: I7f07825167bf8c3236a80e649a6d6805fd6c868b
Reviewed-by: Johan Helsing <johan.helsing@qt.io>

src/plugins/platforms/wayland/qwaylandwindow.cpp

@@ -249,8 +249,10 @@ void QWaylandWindow::reset()
     if (isInitialized())
         destroy();
 
-    if (mFrameCallback)
+    if (mFrameCallback) {
         wl_callback_destroy(mFrameCallback);
+        mFrameCallback = nullptr;
+    }
 }
 
 QWaylandWindow *QWaylandWindow::fromWlSurface(::wl_surface *surface)