ui/clipboard: add asserts for update and request

ui/clipboard: add asserts for update and request

Should an issue like CVE-2023-6683 ever appear again in the future,
it will be more obvious which assumption was violated.

Suggested-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20240124105749.204610-2-f.ebner@proxmox.com>
(cherry picked from commit 9c416582611b7495bdddb4c5456c7acb64b78938)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>