appear segmentfault when calling current_timestamp()

[AricSu]

background :

1. i'm learning BR of TiDB, and use tikv-client to get TSO.
2. however, when i used example of pd.current_timestamp to do, i got an error.

help info :

1. you can access my code at segmentfault path, and run cargo run -- backup table -t 123 -s ./ to reproduce the error
2. after debug, i saw something happened on grpc part of tikv-client.

hopefully, anyone gives me any advise.