protosanitizer: fix panic if enum value is not found

[huww98]

What type of PR is this?

/kind bug

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

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #184

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

protosanitizer: fix panic if enum value is not found

[k8s-ci-robot]

Hi @huww98. Thanks for your PR.

I'm waiting for a kubernetes-csi member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[huww98]

/cc @jsafrane

[carlory]

/ok-to-test

[jsafrane]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: huww98, jsafrane

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