bug#21182: 25.0.50; gnus: every other unread message is marked as read on each nnimap group refresh

[Paul Eggert]

From: Lars Ingebrigtsen <larsi@gnus.org>
> +    (when (nnimap-capability "X-GM-EXT-1")
> +      (push "X-GM-LABELS" params))
>
> So it only adds that to the params if the server supports it...

Suppose the newly-introduced call to nnimap-capability throws an exception or 
signals an error? Could that introduce the symptoms in question?

(I don't really understand the code, just trying to help move this along...)