consumer group: bugfix regex consuming (from v0.8.0+)

v0.8.0 lost initializing the reSeen map in the group consumer, which
causes panics when consuming via regex as a group consumer.

pkg/kgo/consumer_group.go

@@ -137,6 +137,8 @@ func (c *consumer) initGroup() {
 		ctx:    ctx,
 		cancel: cancel,
 
+		reSeen: make(map[string]bool),
+
 		manageDone:       make(chan struct{}),
 		cooperative:      c.cl.cfg.cooperative(),
 		tps:              newTopicsPartitions(),