bpcdump: correct defect that removes all data

bpcdump: correct defect that removes all data

If when looking for space to reclaim for a new backup the ls that scans
for old backups finds nothing, the empty result ends up causing a
recursive removal of the entire contents of the destination directory
(usually a mount point for a partition on a removeable drive).

The correct behavior is to error with a message indicating not enough
space in such situations.