Bugfix: Report an error instead of crashing on a truncated key or config file

Restic crashed with a slice bounds panic when a repository contained a key
file or a config file that was shorter than the encryption overhead, for
example after an upload was interrupted. Since every command has to read a
key file, the whole repository became unusable until the file was removed.
Restic now reports that the file is too short instead.

https://github.com/restic/restic/pull/21977