[server] Only send surface leave if resources are still valid

If a resource is null, because e.g. the surface got already destroyed,
wayland will create an error while marshalling arguments causing
the connected client to in worst case abort.

src/wayland/keyboard_interface.cpp

@@ -118,7 +118,9 @@ void KeyboardInterface::setFocusedSurface(SurfaceInterface *surface, quint32 ser
 {
     Q_D();
     if (d->focusedSurface) {
-        wl_keyboard_send_leave(d->resource, serial, d->focusedSurface->resource());
+        if (d->resource && d->focusedSurface->resource()) {
+            wl_keyboard_send_leave(d->resource, serial, d->focusedSurface->resource());
+        }
         disconnect(d->destroyConnection);
     }
     d->focusedSurface = surface;

src/wayland/pointer_interface.cpp

@@ -116,7 +116,9 @@ void PointerInterface::setFocusedSurface(SurfaceInterface *surface, quint32 seri
     Q_D();
     Q_ASSERT(d->resource);
     if (d->focusedSurface) {
-        wl_pointer_send_leave(d->resource, serial, d->focusedSurface->resource());
+        if (d->resource && d->focusedSurface->resource()) {
+            wl_pointer_send_leave(d->resource, serial, d->focusedSurface->resource());
+        }
         disconnect(d->destroyConnection);
     }
     if (!surface) {