Fix SIGSEGV on output destroy

```
Program terminated with signal SIGSEGV, Segmentation fault.
144                             struct wlr_layer_surface_v1 *layer_surface = surface->layer_surface;
[Current thread is 1 (Thread 0x7f1f7c5b3ac0 (LWP 2473))]
(gdb) bt
```

Add a NULL check in `find_mapped_layer_by_client` like the one in `arrange_surface`.

sway/desktop/layer_shell.c

@@ -140,6 +140,9 @@ static struct sway_layer_surface *find_mapped_layer_by_client(
 		wl_list_for_each (node, &output->layers.shell_overlay->children, link) {
 			struct sway_layer_surface *surface = scene_descriptor_try_get(node,
 				SWAY_SCENE_DESC_LAYER_SHELL);
+			if (!surface) {
+				continue;
+			}
 
 			struct wlr_layer_surface_v1 *layer_surface = surface->layer_surface;
 			struct wl_resource *resource = layer_surface->resource;