There is really no problem here, allow it.

Previously we would assert against a non-zero prefix length.
But I am not sure that all callers really ensured that this
couldn't happen. Anyway, there is no problem we such addresses,
really.

Only we need to make sure that nm_ip4_config_add_dependent_routes()
and nm_ip6_config_add_dependent_routes() don't add prefix routes for
such addresses (which is the case now).