Fix podman system df panic

[JoshVanL]

Fix panic when running podman system df where a node.layer is nil.

Fixes containers/podman#20942

[rhatdan]

/approve
LGTM

[rhatdan]

Thanks @JoshVanL
@vrothberg @Luap99 @umohnani8 @giuseppe @mheon PTAL

[umohnani8]

LGTM

[mheon]

/lgtm
/approve

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JoshVanL, mheon, rhatdan

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[hasan4791]

In which podman version this fix would be expected?
I have 4.9.0 version and still having this issue!!!

As a workaround currently clearing the images with no tags.

podmanImageClean() {
    for i in $(podman images | grep "<none>" | awk '{print $3}');
    do  
        podman rmi -f $i
    done
}

[Luap99]

5.0