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`.

1 files changed, 3 insertions, 0 deletions

diff --git a/sway/desktop/layer_shell.c b/sway/desktop/layer_shell.c
index aca99c97..968b0cdb 100644
--- a/sway/desktop/layer_shell.c
+++ b/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;