backend/drm: Ensure renderer is set to NULL on error

If init_drm_renderer failed, it would destroy the renderer but would not
set it to NULL, leading to use-after-free.

NULL the renderer after destroying it.

backend/drm/renderer.c

@@ -25,6 +25,7 @@ bool init_drm_renderer(struct wlr_drm_backend *drm,
 	if (renderer->allocator == NULL) {
 		wlr_log(WLR_ERROR, "Failed to create allocator");
 		wlr_renderer_destroy(renderer->wlr_rend);
+		renderer->wlr_rend = NULL;
 		return false;
 	}