tikv: fix out-of-index in split-region (pingcap#12265)

store/tikv/split_region.go

@@ -28,6 +28,7 @@ import (
 	"github.com/pingcap/tidb/store/tikv/tikvrpc"
 	"github.com/pingcap/tidb/util"
 	"github.com/pingcap/tidb/util/logutil"
+	"github.com/pingcap/tidb/util/stringutil"
 	"go.uber.org/zap"
 )
 
@@ -154,7 +155,12 @@ func (s *tikvStore) batchSendSingleRegion(bo *Backoffer, batch batch, scatter bo
 	logutil.BgLogger().Info("batch split regions complete",
 		zap.Uint64("batch region ID", batch.regionID.id),
 		zap.Stringer("first at", kv.Key(batch.keys[0])),
-		zap.Stringer("first new region left", logutil.Hex(spResp.Regions[0])),
+		zap.Stringer("first new region left", stringutil.MemoizeStr(func() string {
+			if len(spResp.Regions) == 0 {
+				return ""
+			}
+			return logutil.Hex(spResp.Regions[0]).String()
+		})),
 		zap.Int("new region count", len(spResp.Regions)))
 
 	if !scatter {