Fix PHP Notices when $rate array is returned empty

From times to times, the cron job reported the following PHP Notices:

 PHP Notice:  Undefined index: remaining in /path/to/archive-my-tweets/amt/archiver.php on line 111
 PHP Notice:  Undefined index: limit in /path/to/archive-my-tweets/amt/archiver.php on line 111
 PHP Notice:  Undefined index: remaining in /path/to/archive-my-tweets/amt/archiver.php on line 112

This commit should avoid this to happen when the $rate was not properly retrieved from Twitter