Stop trying to write to address FFFFFFFFFFFFFFF8

get_first_empty_address should return 0 if no address can be fount.

The sub add_link expects get_first_empty_address to return 0 if no empty address is found.

This will prevent trying to add an aldb record to the FFFFFFFFFFFFFFF8 address which clearly doesn't exist.  Instead the user will now get a nice error advising them to scan the link table before trying to sync links.