Fix panic when removing docker images

[feiskyer]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

/kind bug

What this PR does / why we need it:

Dockershim may be panic when deleting images. This PR fixes it by adding a check before using the imageInspect.

Refer kubernetes-sigs/cri-tools#445.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Fix dockershim panic issues when deleting docker images.

/sig node
/kind bug
/assign @Random-Liu @yujuhong

[yujuhong]

Why does docker return nil with no error? Is this a change caused by docker upgrade?

[feiskyer]

Why does docker return nil with no error? Is this a change caused by docker upgrade?

@yujuhong Right, it comes from Docker after upgrading. Anyway, dockershim shouldn't panic.

/retest

[yujuhong]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: feiskyer, yujuhong

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[feiskyer]

/milestone v1.14
/priority critical-urgent

Added to v1.14 since it's a critical bug fixes. It should also be cherry picked to stable releases.