unstable test TestHashInTopN

[wjhuang2016]

Bug Report

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1. Minimal reproduce step (Required)

[2021-12-13T15:03:13.233Z] --- FAIL: TestHashInTopN (9.89s)
[2021-12-13T15:03:13.233Z] panic: runtime error: invalid memory address or nil pointer dereference [recovered]
[2021-12-13T15:03:13.233Z] 	panic: runtime error: invalid memory address or nil pointer dereference
[2021-12-13T15:03:13.233Z] [signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x37e3c92]
[2021-12-13T15:03:13.233Z] 
[2021-12-13T15:03:13.233Z] goroutine 9580 [running]:
[2021-12-13T15:03:13.233Z] testing.tRunner.func1.2(0x3da1b60, 0x6168660)
[2021-12-13T15:03:13.233Z] 	/usr/local/go/src/testing/testing.go:1143 +0x332
[2021-12-13T15:03:13.233Z] testing.tRunner.func1(0xc006291080)
[2021-12-13T15:03:13.233Z] 	/usr/local/go/src/testing/testing.go:1146 +0x4b6
[2021-12-13T15:03:13.233Z] panic(0x3da1b60, 0x6168660)
[2021-12-13T15:03:13.233Z] 	/usr/local/go/src/runtime/panic.go:965 +0x1b9
[2021-12-13T15:03:13.233Z] github.com/pingcap/tidb/executor_test.TestHashInTopN(0xc006291080)
[2021-12-13T15:03:13.233Z] 	/home/jenkins/agent/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/executor/analyze_test.go:770 +0x5f2
[2021-12-13T15:03:13.233Z] testing.tRunner(0xc006291080, 0x43ed508)
[2021-12-13T15:03:13.233Z] 	/usr/local/go/src/testing/testing.go:1193 +0xef
[2021-12-13T15:03:13.233Z] created by testing.(*T).Run
[2021-12-13T15:03:13.233Z] 	/usr/local/go/src/testing/testing.go:1238 +0x2b3
[2021-12-13T15:03:13.233Z] FAIL	github.com/pingcap/tidb/executor	413.186s

https://ci.pingcap.net/blue/rest/organizations/jenkins/pipelines/tidb_ghpr_unit_test/runs/24571/nodes/52/log/?start=0

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

[yudongusa]

@XuHuaiyu it seems this is the fail from executor testing, could you confirm? github.com/pingcap/tidb/executor_test.TestHashInTopN

[Reminiscent]

Related to #30708
We try to disable the parallel for the test to see whether the error still happens.

[Reminiscent]

#30692 This PR has been already disabled the parallel. Maybe we can close this PR until it happens again.

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.