backend/drm: fix segfault in page_flip_handler

Since 5b1b43c68c ("backend/drm: make wlr_drm_plane.{pending,queued,current}_fb
pointers"), current_fb can be NULL if there's no buffer. If current_fb
is not NULL, current_fb->wlr_buf is guaranteed to not be NULL.

Closes: https://github.com/swaywm/wlroots/issues/2634

backend/drm/drm.c

@@ -1484,7 +1484,7 @@ static void page_flip_handler(int fd, unsigned seq,
 	 * data between the GPUs, even if we were using the direct scanout
 	 * interface.
 	 */
-	if (!drm->parent && plane->current_fb->wlr_buf &&
+	if (!drm->parent && plane->current_fb &&
 			wlr_client_buffer_get(plane->current_fb->wlr_buf)) {
 		present_flags |= WLR_OUTPUT_PRESENT_ZERO_COPY;
 	}