[FIX] website: slug: fallback to positive ID when slug appears to contain a missing negative ID

In some rare cases database records have negative IDs,
so the slug URL could look like /foo--20. This could
be mistaken for a slug ending with a `-` and a positive ID.
The latter is not supposed to happned as final hyphens
are stripped by slugify, but has been used in the past
and may be used in old links.