avoid wrongly set startPosition

weed/mq/topic/local_partition.go

@@ -89,12 +89,16 @@ func (p *LocalPartition) Subscribe(clientName string, startPosition log_buffer.M
 			return nil
 		}
 
+		if processedPosition.Time.UnixNano() != 0 {
 			startPosition = processedPosition
+		}
 		processedPosition, isDone, readInMemoryLogErr = p.LogBuffer.LoopProcessLogData(clientName, startPosition, 0, onNoMessageFn, eachMessageFn)
 		if isDone {
 			return nil
 		}
+		if processedPosition.Time.UnixNano() != 0 {
 			startPosition = processedPosition
+		}
 
 		if readInMemoryLogErr == log_buffer.ResumeFromDiskError {
 			continue