touch-input: Don't dereference NULL pointer during full screen fade

If a full screen program is fading out and a touch start happens, it
will result in a NULL pointer dereference when weston_touch_set_focus
tries to derefernce view->surface->resource.

Instead, this patch sets the focus to NULL, which should be the
same as if the program was destroyed during the touch anyway.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=78706
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

desktop-shell/shell.c

@@ -4444,10 +4444,14 @@ fullscreen_binding(struct weston_seat *seat, uint32_t time, uint32_t button, voi
 static void
 touch_move_binding(struct weston_seat *seat, uint32_t time, void *data)
 {
-	struct weston_surface *focus = seat->touch->focus->surface;
+	struct weston_surface *focus;
 	struct weston_surface *surface;
 	struct shell_surface *shsurf;
 
+	if (seat->touch->focus == NULL)
+		return;
+
+	focus = seat->touch->focus->surface;
 	surface = weston_surface_get_main_surface(focus);
 	if (surface == NULL)
 		return;

src/input.c

@@ -1471,8 +1471,14 @@ weston_touch_set_focus(struct weston_seat *seat, struct weston_view *view)
 	}
 
 	if (view) {
-		struct wl_client *surface_client =
-			wl_resource_get_client(view->surface->resource);
+		struct wl_client *surface_client;
+
+		if (!view->surface->resource) {
+			seat->touch->focus = NULL;
+			return;
+		}
+
+		surface_client = wl_resource_get_client(view->surface->resource);
 		move_resources_for_client(focus_resource_list,
 					  &seat->touch->resource_list,
 					  surface_client);