copy: fix nil pointer dereference when checking compression algorithm

[crazy-max]

related to #2047

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x0 pc=0xcbbbf7]

goroutine 1 [running]:
github.com/containers/image/v5/copy.(*imageCopier).compareImageDestinationManifestEqual(0xc0000c3860, {0x1b88850, 0xc000044158}, 0xc00001cc08)
        /home/crazymax/github.com/containers/image/[email protected]/copy/single.go:384 +0x677
github.com/containers/image/v5/copy.(*copier).copySingleImage(0xc000427b80, {0x1b88850, 0xc000044158}, 0xc00048dc80, 0xc00001cc08, {0x4?, 0x0?, 0x0?})
        /home/crazymax/github.com/containers/image/[email protected]/copy/single.go:194 +0xde5
github.com/containers/image/v5/copy.(*copier).copyMultipleImages(0xc000427b80, {0x1b88850, 0xc000044158})
        /home/crazymax/github.com/containers/image/[email protected]/copy/multiple.go:242 +0x1149
github.com/containers/image/v5/copy.Image({0x1b88850, 0xc000044158}, 0xc0005670f8, {0x1b8e7e0, 0xc0002a8c60}, {0x1b8e660?, 0xc00071e840?}, 0x0?)
        /home/crazymax/github.com/containers/image/[email protected]/copy/copy.go:337 +0x1485

cc @flouthoc @mtrmac

[flouthoc]

Thanks for fixing this. I am just wondering what is the MIME type in your case ? looks some unexpected MIME type ( anyways nil should be handled so thanks for fixing this )

[crazy-max]

Thanks for fixing this. I am just wondering what is the MIME type in your case ? looks some unexpected MIME type ( anyways nil should be handled so thanks for fixing this )

It was for a layer part of an attestation manifest dockereng/packaging:docker-engine-v24.0.4: https://explore.ggcr.dev/?image=dockereng/packaging@sha256:1e06504de44779bb34895d780f1705c8e156255ec477042146555cf32d98034f&mt=application%2Fvnd.oci.image.manifest.v1%2Bjson&size=566

[crazy-max]

You can create such manifest with BuildKit: https://docs.docker.com/build/attestations/attestation-storage/

[flouthoc]

@crazy-max Thanks for sharing this.

[mtrmac]

LGTM. Thanks!