Bugfix: Fixed unexpected 'pack file cannot be listed' error

Due to a regression introduced in 0.8.2, the `rebuild-index` and `prune`
commands failed to read pack files with size of 587, 588, 589 or 590 bytes.

https://github.com/restic/restic/issues/1633
https://github.com/restic/restic/pull/1635
