Abort if neither --database-dir nor --base-dir is given.

Abort if neither --database-dir nor --base-dir is given.

If --database-dir is not given, adu concatenates the database root
directory (which defaults to /var/lib/adu) with the value of the
--base-dir argument. However, if --base-dir is not given either,
base_dir_arg is NULL, and the concatenated string becomes the result
is /var/lib/adu(null), at least on glibc.

With this patch applied, adu handles this case more gracefully: it
prints an error message and aborts early.