shell: Only assign focus on click if there is a pointer focus surface

If we don't have a background image from the desktop-shell client or the
pointer for some other reason doesn't have a focus we trigger a
segfault as we try to deref the seat->pointer->focus NULL pointer.

https://bugs.freedesktop.org/show_bug.cgi?id=73066

clients/desktop-shell.c

@@ -765,7 +765,7 @@ background_configure(void *data,
 	struct background *background =
 		(struct background *) window_get_user_data(window);
 
-	widget_schedule_resize(background->widget, width, height);
+	widget_schedule_resize(background->widget, width - 256, height);
 }
 
 static void

desktop-shell/shell.c

@@ -4216,6 +4216,8 @@ click_to_activate_binding(struct weston_seat *seat, uint32_t time, uint32_t butt
 {
 	if (seat->pointer->grab != &seat->pointer->default_grab)
 		return;
+	if (seat->pointer->focus == NULL)
+		return;
 
 	activate_binding(seat, data, seat->pointer->focus->surface);
 }