backend/headless: fix regression with common renderer and allocator

Commit c82f375 ("backend/headless: use common renderer and allocator")
changed the error handling for backend allocator failure cases.

Let's fix the issue by moving the wl_signal_add() call later. This was
also done earlier in wlr_headless_backend_create_with_renderer().

Fixes: c82f375 ("backend/headless: use common renderer and allocator")

backend/headless/backend.c

@@ -134,14 +134,18 @@ static bool backend_init(struct wlr_headless_backend *backend,
 	} else {
 		backend->parent_renderer = renderer;
 		backend->parent_renderer_destroy.notify = handle_renderer_destroy;
-		wl_signal_add(&renderer->events.destroy, &backend->parent_renderer_destroy);
 	}
 
 	if (backend_get_allocator(&backend->backend) == NULL) {
 		wlr_log(WLR_ERROR, "Failed to create allocator");
 		return false;
 	}
 
+	if (renderer != NULL) {
+		wl_signal_add(&renderer->events.destroy,
+			&backend->parent_renderer_destroy);
+	}
+
 	backend->display_destroy.notify = handle_display_destroy;
 	wl_display_add_destroy_listener(display, &backend->display_destroy);