etcd-tester: fix wrong error checking

Hash method returns either (nil, err) or (Hash, nil).
The current error checking is wrong. It only needs to check
the error is either nil or non-nil.

This causes panic in https://github.com/coreos/etcd/issues/4463
by allowing the case when resp is nil, but err is not nil.

tools/functional-tester/etcd-tester/tester.go

@@ -235,7 +235,7 @@ func (c *cluster) getKVHash() (map[string]int64, error) {
 		kvc := pb.NewKVClient(conn)
 		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 		resp, err := kvc.Hash(ctx, &pb.HashRequest{})
-		if resp != nil && err != nil {
+		if err != nil {
 			return nil, err
 		}
 		cancel()