avoid nil stopChan

weed/mq/sub_coordinator/consumer_group.go

@@ -24,6 +24,7 @@ func NewConsumerGroup(t *mq_pb.Topic, reblanceSeconds int32, filerClientAccessor
 		topic:                  topic.FromPbTopic(t),
 		ConsumerGroupInstances: cmap.New[*ConsumerGroupInstance](),
 		filerClientAccessor:    filerClientAccessor,
+		stopCh:                 make(chan struct{}),
 	}
 	if conf, err := cg.filerClientAccessor.ReadTopicConfFromFiler(cg.topic); err == nil {
 		var partitions []topic.Partition