middleware: fix waitForLeader in redirector panic

[HuSharp]

What problem does this PR solve?

Issue Number: Close #8406

What is changed and how does it work?

serverLoopContext is initialized here.

pd/server/server.go

Lines 660 to 661 in c50993b

	func (s *Server) startServerLoop(ctx context.Context) {
	s.serverLoopCtx, s.serverLoopCancel = context.WithCancel(ctx)

Probably called after waitForLeader.

Check List

Tests

• No code

Release note

middleware: fix waitForLeader in redirector panic

[HuSharp]

/check-issue-triage-complete

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.19%. Comparing base (cbba49a) to head (96bb8b0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8407      +/-   ##
==========================================
- Coverage   77.24%   77.19%   -0.06%     
==========================================
  Files         471      471              
  Lines       61700    61700              
==========================================
- Hits        47662    47630      -32     
- Misses      10463    10487      +24     
- Partials     3575     3583       +8     

Flag	Coverage Δ
unittests	77.19% <100.00%> (-0.06%)	⬇️

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JmPotato, rleungx

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-17 03:31:12.914319568 +0000 UTC m=+411094.905261036: ☑️ agreed by JmPotato.
• 2024-07-17 03:33:46.284085868 +0000 UTC m=+411248.275027346: ☑️ agreed by rleungx.