Controller: Fix panic in alternative backend merging.

[chengjoey]

What this PR does / why we need it:

fix nginx controller panic when alternative backend not in upstreams

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• CVE Report (Scanner found CVE and adding report)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only

Which issue/s this PR fixes

#11661

How Has This Been Tested?

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added unit and/or e2e tests to cover my changes.
• All new and existing tests passed.

[k8s-ci-robot]

Hi @chengjoey. Thanks for your PR.

I'm waiting for a kubernetes 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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[netlify]

✅ Deploy Preview for kubernetes-ingress-nginx canceled.

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[chengjoey]

cc @rsafonseca @rikatz

[chengjoey]

/assign @Gacko

[Gacko]

/assign

Thanks, I remind this one. Looks good to me, gonna have a detailed look in a few! :)

[Gacko]

I think you forgot to add the check in one place, I added it for you.

[Gacko]

/triage accepted
/kind bug
/priority important-soon
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chengjoey, Gacko

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• OWNERS [Gacko]

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[Gacko]

/cherry-pick release-1.10

[k8s-infra-cherrypick-robot]

@Gacko: once the present PR merges, I will cherry-pick it on top of release-1.10 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.10

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[Gacko]

/cherry-pick release-1.11

[k8s-infra-cherrypick-robot]

@Gacko: once the present PR merges, I will cherry-pick it on top of release-1.11 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.11

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[k8s-infra-cherrypick-robot]

@Gacko: new pull request created: #11793

In response to this:

/cherry-pick release-1.10

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[k8s-infra-cherrypick-robot]

@Gacko: new pull request created: #11794

In response to this:

/cherry-pick release-1.11

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.