Guard against nil id and missing page in delete-page to prevent
broken changes from being sent to the server. This can happen due
to a race condition where the page is no longer present in the
pages-index. Also add assertion in changes-builder/del-page as
defense-in-depth.
Signed-off-by: Andrey Antukh <niwi@niwi.nz>
6bf7c33c43