Bugfix: `rebuild-index` failed if an index file was damaged

Previously, the `rebuild-index` command would fail with an error if an index
file was damaged or truncated. This has now been fixed.

On older restic versions, a (slow) workaround is to use
`rebuild-index --read-all-packs` or to manually delete the damaged index.

https://github.com/restic/restic/pull/3488
