Potential bug related to error checking.

[vembacher-unbasical]

I believe I found a potential bug in this piece of code:

https://github.com/containers/tar-diff/blob/f08980f165b5b4e50c466a695b906b9b28069c6c/pkg/tar-diff/diff.go#L298-L302

Is returning nil instead of err intentional here?

[vembacher-unbasical]

This can cause issues (that are incredibly hard to track down) when this piece of code fails:

https://github.com/containers/tar-diff/blob/f08980f165b5b4e50c466a695b906b9b28069c6c/pkg/tar-diff/analysis.go#L327-L330