*: fix data race in the file of prepare_test.go

[zimulala]

What have you changed? (mandatory)

Fix data race in the file of prepare_test.go

Read at 0x0000028eb742 by goroutine 35:
  github.com/pingcap/tidb/plan.tryFastPlan()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/plan/point_get_plan.go:117 +0x3e
  github.com/pingcap/tidb/plan.Optimize()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/plan/optimizer.go:62 +0x77
  github.com/pingcap/tidb/executor.(*Compiler).Compile()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/executor/compiler.go:48 +0x221
  github.com/pingcap/tidb/session.(*session).execute()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/session/session.go:807 +0x80f
  github.com/pingcap/tidb/session.(*session).Execute()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/session/session.go:765 +0x79
  github.com/pingcap/tidb/ddl/util.CompleteDeleteRange()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/ddl/util/util.go:94 +0x1f9
  github.com/pingcap/tidb/ddl.(*delRange).doTask()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/ddl/delete_range.go:200 +0x432
  github.com/pingcap/tidb/ddl.(*delRange).doDelRangeWork()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/ddl/delete_range.go:151 +0x541
  github.com/pingcap/tidb/ddl.(*delRange).doTask()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/ddl/delete_range.go:200 +0x432
  github.com/pingcap/tidb/ddl.(*delRange).doDelRangeWork()
...

Previous write at 0x0000028eb742 by goroutine 749:
  github.com/pingcap/tidb/executor_test.(*testSuite).TestPreparedNullParam.func1()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/executor/prepared_test.go:229 +0x3a
  github.com/pingcap/tidb/executor_test.(*testSuite).TestPreparedNullParam()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/executor/prepared_test.go:259 +0x5d3
  github.com/pingcap/tidb/executor_test.(*testSuite).TestPreparedNullParam()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/executor/prepared_test.go:256 +0x544
  github.com/pingcap/tidb/executor_test.(*testSuite).TestPreparedNullParam()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/executor/prepared_test.go:253 +0x4ee
  github.com/pingcap/tidb/executor_test.(*testSuite).TestPreparedNullParam()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/executor/prepared_test.go:250 +0x43b
  github.com/pingcap/tidb/executor_test.(*testSuite).TestPreparedNullParam()
      /home/jenkins/workspace/tidb_ghpr_unit_test/go/src/github.com/pingcap/tidb/executor/prepared_test.go:249 +0x404
  github.com/pingcap/tidb/executor_test.(*testSuite).TestPreparedNullParam()
    

What is the type of the changes? (mandatory)

• Improvement (non-breaking change which is an improvement to an existing feature)

How has this PR been tested? (mandatory)

Yes.

Does this PR affect documentation (docs/docs-cn) update? (mandatory)

No.

Does this PR affect tidb-ansible update? (mandatory)

No.

Does this PR need to be added to the release notes? (mandatory)

No.

Refer to a related PR or issue link (optional)

Benchmark result if necessary (optional)

Add a few positive/negative examples (optional)

[zimulala]

/run-all-tests

[jackysp]

LGTM

[winkyao]

LGTM

[jackysp]

LGTM