fix panic on publish with not connected broker

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

kgo.go

@@ -243,6 +243,12 @@ func (k *kBroker) Publish(ctx context.Context, topic string, msg *broker.Message
 }
 
 func (k *kBroker) publish(ctx context.Context, msgs []*broker.Message, opts ...broker.PublishOption) error {
+	k.RLock()
+	if !k.connected {
+		k.RUnlock()
+		return broker.ErrNotConnected
+	}
+	k.RUnlock()
 	options := broker.NewPublishOptions(opts...)
 	records := make([]*kgo.Record, 0, len(msgs))
 	var errs []string