export-dmabuf: Keep frame->output = NULL until frame is ready

This fixes a crash that happens when a client requests a frame on a
backend that does not implement export_dmabuf.

An assertion fails with the message:
sway: types/wlr_output.c:777: wlr_output_lock_attach_render: Assertion
`output->attach_render_locks > 0' failed.

types/wlr_export_dmabuf_v1.c

@@ -98,7 +98,6 @@ static void manager_handle_capture_output(struct wl_client *client,
 		return;
 	}
 	frame->manager = manager;
-	frame->output = output;
 	wl_list_init(&frame->output_precommit.link);
 
 	uint32_t version = wl_resource_get_version(manager_resource);
@@ -129,6 +128,8 @@ static void manager_handle_capture_output(struct wl_client *client,
 		return;
 	}
 
+	frame->output = output;
+
 	wlr_output_lock_attach_render(frame->output, true);
 	if (overlay_cursor) {
 		wlr_output_lock_software_cursors(frame->output, true);