drainer panics because of tikv-client bug

[lichunzhu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.
   Run drainer to replicate some data.

2. What did you expect to see?
   Drainer replicates data successfully.

3. What did you see instead?

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x2b0 pc=0x1d18be6]

goroutine 265296850 [running]:
[google.golang.org/grpc.(*ClientConn).GetState(...)](http://google.golang.org/grpc.(*ClientConn).GetState(...))
        /root/go/pkg/mod/[google.golang.org/grpc@v1.44.0/clientconn.go:527](http://google.golang.org/grpc@v1.44.0/clientconn.go:527)
[github.com/tikv/client-go/v2/internal/client.(*RPCClient).SendRequest](http://github.com/tikv/client-go/v2/internal/client.(*RPCClient).SendRequest)(0xc009596680, {0x4023728, 0xc018232b80}, {0xc00d9b80d8, 0x11}, 0xc005a0f680, 0x6fc23ac00?)
        /root/go/pkg/mod/[github.com/tikv/client-go/v2@v2.0.1-0.20220311043619-5042c6f2aaa6/internal/client/client.go:410](http://github.com/tikv/client-go/v2@v2.0.1-0.20220311043619-5042c6f2aaa6/internal/client/client.go:410) +0x666
[github.com/tikv/client-go/v2/internal/client.interceptedClient.SendRequest](http://github.com/tikv/client-go/v2/internal/client.interceptedClient.SendRequest)({{0x4013b98?, 0xc009596680?}}, {0x4023728, 0xc018232b80}, {0xc00d9b80d8, 0x11}, 0x18bf128?, 0x6fc23ac00)
        /root/go/pkg/mod/[github.com/tikv/client-go/v2@v2.0.1-0.20220311043619-5042c6f2aaa6/internal/client/client_interceptor.go:42](http://github.com/tikv/client-go/v2@v2.0.1-0.20220311043619-5042c6f2aaa6/internal/client/client_interceptor.go:42) +0x186
[github.com/tikv/client-go/v2/internal/client.reqCollapse.SendRequest](http://github.com/tikv/client-go/v2/internal/client.reqCollapse.SendRequest)({{0x4015b50?, 0xc0115a4c30?}}, {0x4023728, 0xc018232b80}, {0xc00d9b80d8, 0x11}, 0x397d300?, 0x7f76d4b0a101?)
        /root/go/pkg/mod/[github.com/tikv/client-go/v2@v2.0.1-0.20220311043619-5042c6f2aaa6/internal/client/client_collapse.go:74](http://github.com/tikv/client-go/v2@v2.0.1-0.20220311043619-5042c6f2aaa6/internal/client/client_collapse.go:74) +0xd6
[github.com/tikv/client-go/v2/tikv.(*KVStore).updateSafeTS.func1](http://github.com/tikv/client-go/v2/tikv.(*KVStore).updateSafeTS.func1)({0x4023728, 0xc018232b80}, 0xc0057086c0?, 0x2a, {0xc00d9b80d8, 0x11})
        /root/go/pkg/mod/[github.com/tikv/client-go/v2@v2.0.1-0.20220311043619-5042c6f2aaa6/tikv/kv.go:540](http://github.com/tikv/client-go/v2@v2.0.1-0.20220311043619-5042c6f2aaa6/tikv/kv.go:540) +0x195
created by [github.com/tikv/client-go/v2/tikv.(*KVStore).updateSafeTS](http://github.com/tikv/client-go/v2/tikv.(*KVStore).updateSafeTS)
        /root/go/pkg/mod/[github.com/tikv/client-go/v2@v2.0.1-0.20220311043619-5042c6f2aaa6/tikv/kv.go:538](http://github.com/tikv/client-go/v2@v2.0.1-0.20220311043619-5042c6f2aaa6/tikv/kv.go:538) +0x87

5. Please provide the relate downstream type and version of drainer.
   (run drainer -V in terminal to get drainer's version)
   master