BR panics when backing up to an S3-compatible storage that does not implement object locking

[kennytm]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Backup to an external S3-compatible storage on Huawei OBS

br backup full -s s3://bucket/prefix?endpoint=https://obs.cn-east-3.myhuaweicloud.com/ ...

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

Backup success

3. What did you see instead (Required)

Panics with this stack trace:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x31428da]goroutine 1 [running]:
github.com/pingcap/tidb/br/pkg/storage.(*S3Storage).isObjectLockEnabled(0xc0014222a0)
    /home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/br/pkg/storage/s3.go:460 +0x29a
github.com/pingcap/tidb/br/pkg/storage.newS3Storage(0xc00024e3c0, 0xc00107d4f8)
    /home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/br/pkg/storage/s3.go:403 +0xe25
github.com/pingcap/tidb/br/pkg/storage.New({0x5266050?, 0xc000646340?}, 0xc00121ee48?, 0x1880727?)
    /home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/br/pkg/storage/storage.go:176 +0x75
github.com/pingcap/tidb/br/pkg/backup.(*Client).SetStorage(0xc000eac740, {0x5266050?, 0xc000646340?}, 0x4b4aba6?, 0x5266050?)
    /home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/br/pkg/backup/client.go:223 +0x5b
github.com/pingcap/tidb/br/pkg/backup.(*Client).SetStorageAndCheckNotInUse(0xc000eac740, {0x5266050, 0xc000646340}, 0xc000e3a900?, 0x0?)
    /home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/br/pkg/backup/client.go:192 +0x2c
github.com/pingcap/tidb/br/pkg/task.RunBackup({0x5266050, 0xc000d0e540}, {0x527e410?, 0x7a79bd8?}, {0x4b33732, 0xb}, 0xc00092c380)
    /home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/br/pkg/task/backup.go:332 +0x735
main.runBackupCommand(0xc000d94c80, {0x4b33732, 0xb})
    /home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/br/cmd/br/backup.go:48 +0x418
main.newFullBackupCommand.func1(0xc000d94c80?, {0xc000d5c840?, 0xb?, 0xb?})
    /home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/br/cmd/br/backup.go:117 +0x25
github.com/spf13/cobra.(*Command).execute(0xc000d94c80, {0xc00013c030, 0xb, 0xb})
    /go/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:872 +0x694
github.com/spf13/cobra.(*Command).ExecuteC(0xc000d19180)
    /go/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:990 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
    /go/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:918
main.main()
    /home/jenkins/agent/workspace/build-common/go/src/github.com/pingcap/br/br/cmd/br/main.go:57 +0x33c 

4. What is your TiDB version? (Required)

v6.4.0

[kennytm]

tidb/br/pkg/storage/s3.go

Lines 459 to 463 in ac0d36b

	if resp.ObjectLockConfiguration != nil {
	if s3.ObjectLockEnabledEnabled == *resp.ObjectLockConfiguration.ObjectLockEnabled {
	return true
	}
	}

The code only checked whether resp.ObjectLockConfiguration is nil, but not if resp.ObjectLockConfiguration.ObjectLockEnabled is nil.

The code should use aws.StringValue(x) instead of *x.