jetstream: ordered consumer: handle closed subscriptions gracefully

In `orderedConsumer.reset()`, we return from the `retryWithBackoff()`
function with `false, nil` in case the subscription is closed. In this case,
the `var cons Consumer` is still nil, and we crash in the attempt to cast it
to a `pullConsumer` later on.

Fix this by catching that `nil` case explicitly.

jetstream/ordered.go

@@ -539,6 +539,9 @@ func (c *orderedConsumer) reset() error {
 	if err != nil {
 		return err
 	}
+	if cons == nil {
+		return nats.ErrSubscriptionClosed
+	}
 	c.currentConsumer = cons.(*pullConsumer)
 	return nil
 }