jetstream: ordered consumer: handle closed subscriptions gracefully

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

Fix this by catching the `nil` case explicitly.

Signed-off-by: Daniel Mack <[email protected]>

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
 }