pd-api-bench: fix the gosec check

[JmPotato]

What problem does this PR solve?

Issue Number: ref #4399.

main.go:165:31: G602: Potentially accessing slice out of bounds (gosec)
	err = cases.InitCluster(ctx, pdClis[0], httpClis[0])
	                             ^

What is changed and how does it work?

Fix the gosec check warning.

Check List

Tests

• Unit test
• Integration test

Release note

None.

[ti-chi-bot]

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This pull request has been approved by:

• lhy1024

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[JmPotato]

/merge

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

Commit hash: a49b71a