Bugfix: Don't generate negative UIDs and GIDs in tar files from `dump`

When using a 32-bit build of restic, the `dump` command could in some cases
create tar files containing negative UIDs and GIDs, which cannot be read by
GNU tar. This corner case especially applies to backups from stdin on Windows.

This is now fixed such that `dump` creates valid tar files in these cases too.

https://github.com/restic/restic/issues/4103
https://github.com/restic/restic/pull/4104