DATA RACE in the TestInitStatsVer2

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

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

### 3. What did you see instead (Required)

```
==================
WARNING: DATA RACE
Write at 0x00c007b7ea33 by goroutine 48:
github.com/pingcap/tidb/session.(*session).ExecuteInternal()
session/session.go:1612 +0x118
github.com/pingcap/tidb/statistics/handle.(*Handle).initTopNCountSum()
statistics/handle/bootstrap.go:343 +0x205
github.com/pingcap/tidb/statistics/handle.(*Handle).initStatsHistograms4Chunk()
statistics/handle/bootstrap.go:146 +0xb7a
github.com/pingcap/tidb/statistics/handle.(*Handle).initStatsHistograms()
statistics/handle/bootstrap.go:187 +0x2a4
github.com/pingcap/tidb/statistics/handle.(*Handle).InitStats()
statistics/handle/bootstrap.go:420 +0x275
statistics/handle/handletest/handletest_test.TestInitStatsVer2()
statistics/handle/handletest/handle_test.go:478 +0x264
github.com/pingcap/tidb/domain.(*Domain).rebuildSysVarCache()
domain/sysvar_cache.go:146 +0x8c4
github.com/pingcap/tidb/domain.(*Domain).LoadSysVarCacheLoop()
domain/domain.go:1470 +0xa8
github.com/pingcap/tidb/session.BootstrapSession()
session/session.go:3314 +0x6d3
github.com/pingcap/tidb/domain.(*Domain).GetSessionCache()
domain/sysvar_cache.go:62 +0x5c
github.com/pingcap/tidb/session.(*session).loadCommonGlobalVariablesIfNeeded()
session/session.go:3656 +0x104
github.com/pingcap/tidb/session.(*session).ExecuteStmt()
session/session.go:2093 +0x145
github.com/pingcap/tidb/session.(*session).ExecuteInternal()
session/session.go:1628 +0x31b
github.com/pingcap/tidb/domain.(*Domain).LoadPrivilegeLoop()
domain/domain.go:1414 +0x130
github.com/pingcap/tidb/session.BootstrapSession()
session/session.go:3307 +0x684
github.com/pingcap/tidb/testkit.bootstrap()
testkit/mockstore.go:85 +0xac
github.com/pingcap/tidb/testkit.CreateMockStoreAndDomain()
testkit/mockstore.go:70 +0xe9
statistics/handle/handletest/handletest_test.TestInitStatsVer2()
statistics/handle/handletest/handle_test.go:462 +0x58
testing.tRunner()
GOROOT/src/testing/testing.go:1576 +0x216
testing.(*T).Run.func1()
GOROOT/src/testing/testing.go:1629 +0x47
Previous read at 0x00c007b7ea33 by goroutine 30154:
github.com/pingcap/tidb/distsql.(*selectResult).fetchResp.func1()
distsql/select_result.go:165 +0x90
runtime.deferreturn()
GOROOT/src/runtime/panic.go:476 +0x32
github.com/pingcap/tidb/distsql.(*selectResult).Next()
distsql/select_result.go:262 +0x198
github.com/pingcap/tidb/executor.(*tableResultHandler).nextChunk()
executor/table_reader.go:516 +0x1a5
github.com/pingcap/tidb/executor.(*TableReaderExecutor).Next()
executor/table_reader.go:255 +0x350
github.com/pingcap/tidb/executor.Next()
executor/executor.go:324 +0x326
github.com/pingcap/tidb/executor.(*projectionInputFetcher).run()
executor/projection.go:388 +0x5e4
github.com/pingcap/tidb/executor.(*ProjectionExec).prepare.func1()
executor/projection.go:277 +0x58
Goroutine 48 (running) created at:
testing.(*T).Run()
GOROOT/src/testing/testing.go:1629 +0x805
testing.runTests.func1()
GOROOT/src/testing/testing.go:2036 +0x8d
testing.tRunner()
GOROOT/src/testing/testing.go:1576 +0x216
testing.runTests()
GOROOT/src/testing/testing.go:2034 +0x87c
testing.(*M).Run()
GOROOT/src/testing/testing.go:1906 +0xb44
go.uber.org/goleak.VerifyTestMain()
external/org_uber_go_goleak/testmain.go:53 +0x70
statistics/handle/handletest/handletest_test.TestMain()
statistics/handle/handletest/main_test.go:32 +0x435
main.main()
bazel-out/k8-fastbuild/bin/statistics/handle/handletest/handletest_test_/testmain.go:246 +0x5e8
Goroutine 30154 (finished) created at:
github.com/pingcap/tidb/executor.(*ProjectionExec).prepare()
executor/projection.go:277 +0xd59
github.com/pingcap/tidb/executor.(*ProjectionExec).parallelExecute()
executor/projection.go:212 +0xd6
github.com/pingcap/tidb/executor.(*ProjectionExec).Next()
executor/projection.go:185 +0xcc
github.com/pingcap/tidb/executor.Next()
executor/executor.go:324 +0x326
github.com/pingcap/tidb/executor.(*ExecStmt).next()
executor/adapter.go:1211 +0x89
github.com/pingcap/tidb/executor.(*recordSet).Next()
executor/adapter.go:149 +0x164
github.com/pingcap/tidb/session.(*execStmtResult).Next()
:1 +0x76
github.com/pingcap/tidb/statistics/handle.(*Handle).initStatsHistograms()
statistics/handle/bootstrap.go:180 +0x2d5
github.com/pingcap/tidb/statistics/handle.(*Handle).InitStats()
statistics/handle/bootstrap.go:420 +0x275
statistics/handle/handletest/handletest_test.TestInitStatsVer2()
statistics/handle/handletest/handle_test.go:478 +0x264
github.com/pingcap/tidb/domain.(*Domain).rebuildSysVarCache()
domain/sysvar_cache.go:146 +0x8c4
github.com/pingcap/tidb/domain.(*Domain).LoadSysVarCacheLoop()
domain/domain.go:1470 +0xa8
github.com/pingcap/tidb/session.BootstrapSession()
session/session.go:3314 +0x6d3
github.com/pingcap/tidb/domain.(*Domain).GetSessionCache()
domain/sysvar_cache.go:62 +0x5c
github.com/pingcap/tidb/session.(*session).loadCommonGlobalVariablesIfNeeded()
session/session.go:3656 +0x104
github.com/pingcap/tidb/session.(*session).ExecuteStmt()
session/session.go:2093 +0x145
github.com/pingcap/tidb/session.(*session).ExecuteInternal()
session/session.go:1628 +0x31b
github.com/pingcap/tidb/domain.(*Domain).LoadPrivilegeLoop()
domain/domain.go:1414 +0x130
github.com/pingcap/tidb/session.BootstrapSession()
session/session.go:3307 +0x684
github.com/pingcap/tidb/testkit.bootstrap()
testkit/mockstore.go:85 +0xac
github.com/pingcap/tidb/testkit.CreateMockStoreAndDomain()
testkit/mockstore.go:70 +0xe9
statistics/handle/handletest/handletest_test.TestInitStatsVer2()
statistics/handle/handletest/handle_test.go:462 +0x58
testing.tRunner()
GOROOT/src/testing/testing.go:1576 +0x216
testing.(*T).Run.func1()
GOROOT/src/testing/testing.go:1629 +0x47
==================
```
### 4. What is your TiDB version? (Required)