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>
diff --git a/desktop-shell/shell.c b/desktop-shell/shell.c
index bf52b93..ad6750d 100644
--- a/desktop-shell/shell.c
+++ b/desktop-shell/shell.c
@@ -4444,10 +4444,14 @@
 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;