ctl: fix https client panic

[okJiang]

What problem does this PR solve?

Issue Number: Close #8237, Ref #7300

What is changed and how does it work?

fix panic when call pd-ctl cluster with tls

Check List

Tests

• Unit test

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• rleungx

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[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 6 lines in your changes missing coverage. Please review.

Project coverage is 77.34%. Comparing base (492a873) to head (3b0a7ed).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8239      +/-   ##
==========================================
+ Coverage   77.30%   77.34%   +0.03%     
==========================================
  Files         471      471              
  Lines       61377    61365      -12     
==========================================
+ Hits        47446    47460      +14     
+ Misses      10370    10349      -21     
+ Partials     3561     3556       -5     

Flag	Coverage Δ
unittests	77.34% <71.42%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[HuSharp]

/merge

[ti-chi-bot]

@HuSharp: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: bba4466

[okJiang]

/run-all-tests

[JmPotato]

/check-dco

[wuhuizuo]

/check-dco

[JmPotato]

/merge

[ti-chi-bot]

@JmPotato: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

If you have any questions about the PR merge process, please refer to pr process.

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 ti-community-infra/tichi repository.

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: ff1b88a

[wuhuizuo]

/check-dco

[ti-chi-bot]

Thanks for your pull request. Before we can look at it, you'll need to add a 'DCO signoff' to your commits.

📝 Please follow instructions in the contributing guide to update your commits with the DCO

Full details of the Developer Certificate of Origin can be found at developercertificate.org.

The list of commits missing DCO signoff:

• 3b0a7ed Merge branch 'master' into pdctl-tls

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[JmPotato]

/cherry-pick release-8.1

[ti-chi-bot]

@JmPotato: new pull request created to branch release-8.1: #8247.

In response to this:

/cherry-pick release-8.1

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