Avoid a nil pointer

With many MaxInFlight, we attempted to close the connection
even after it was closed and nil.

It's a hacky fix but this whole area will be redesigned

pkg/amqp10_client/publisher.go

@@ -263,5 +263,7 @@ func (p *Amqp10Publisher) Send() error {
 
 func (p *Amqp10Publisher) Stop(reason string) {
 	log.Debug("closing connection", "id", p.Id, "reason", reason)
-	_ = p.Connection.Close()
+	if p.Connection != nil {
+		_ = p.Connection.Close()
+	}
 }