sorter: fix the bug that value can be corrupted when accessing sorter (…

…pingcap#11548) (pingcap#11568)

close pingcap#6367

cdc/processor/sourcemanager/engine/pebble/event_sorter.go

@@ -318,14 +318,16 @@ func (s *EventIter) Next() (event *model.PolymorphicEvent, pos engine.Position,
 	valid := s.iter != nil && s.iter.Valid()
 	var value []byte
 	for valid {
-		nextStart := time.Now()
-		value, valid = s.iter.Value(), s.iter.Next()
-		s.nextDuration.Observe(time.Since(nextStart).Seconds())
-
+		value = s.iter.Value()
 		event = &model.PolymorphicEvent{}
 		if _, err = s.serde.Unmarshal(event, value); err != nil {
 			return
 		}
+
+		nextStart := time.Now()
+		valid = s.iter.Next()
+		s.nextDuration.Observe(time.Since(nextStart).Seconds())
+
 		if s.headItem != nil {
 			break
 		}