option: Fix to reject invalid and overflowing numbers

parse_option_number() fails to check for these errors after
strtoull().  Has always been broken.  Fix that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1487708048-2131-10-git-send-email-armbru@redhat.com>
