tools: fix panic issue in tso bench (#8449)

close #8448

tools: fix panic issue in tso bench

Signed-off-by: nolouch <nolouch@gmail.com>

Co-authored-by: ti-chi-bot[bot] <108142056+ti-chi-bot[bot]@users.noreply.github.com>

tools/pd-tso-bench/main.go

@@ -386,20 +386,27 @@ func reqWorker(ctx context.Context, pdClients []pd.Client, clientIdx int, durCh
 			var ticker *time.Ticker
 			if *maxTSOSendIntervalMilliseconds > 0 {
 				sleepBeforeGetTS := time.Duration(rand.Intn(*maxTSOSendIntervalMilliseconds)) * time.Millisecond
-				ticker = time.NewTicker(sleepBeforeGetTS)
-				select {
-				case <-reqCtx.Done():
-				case <-ticker.C:
-					totalSleepBeforeGetTS += sleepBeforeGetTS
+				if sleepBeforeGetTS > 0 {
+					ticker = time.NewTicker(sleepBeforeGetTS)
+					select {
+					case <-reqCtx.Done():
+					case <-ticker.C:
+						totalSleepBeforeGetTS += sleepBeforeGetTS
+					}
 				}
 			}
 			_, _, err = pdCli.GetLocalTS(reqCtx, *dcLocation)
 			if errors.Cause(err) == context.Canceled {
-				ticker.Stop()
+				if ticker != nil {
+					ticker.Stop()
+				}
+
 				return
 			}
 			if err == nil {
-				ticker.Stop()
+				if ticker != nil {
+					ticker.Stop()
+				}
 				break
 			}
 			log.Error(fmt.Sprintf("%v", err))