Correctly handle most recent backup if it's older than 24hs.

Code was assuming that you'd run backups hourly, eg, so you'd have
many backups within the last 24 hours. Code assumed that by the time
you found a backup more than 24h old, you'd already have a value for
$prev set. In the event that your most recent backup is more than 24
hours old, $prev would not be set and bash would get an error "unary
operator expected" when it tried to compare the day of the newest
backup to the empty $prev value.

I fix this by setting a default value for $prev, one that cannot be
mistaken for an existing backup because it does not represent a valid
date. This ensures that the most recent backup is correctly preserved
regardless of it's age.