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In a GUI app, when interactive userpass input begins, the Ldisc acquires a reference to a prompts_t. If something bad happens to the SSH connection (e.g. unexpected server-side closure), then all the SSH layers will be destroyed, including freeing that prompts_t. So the Ldisc will have a stale reference to it, which it might potentially use. To fix that, I've arranged a back-pointer so that prompts_t itself can find the Ldisc's reference to it, and NULL it out on free. So now, whichever of a prompts_t and an Ldisc is freed first, the link between them should be cleanly broken. (I'm not 100% sure this is absolutely necessary, in the sense of whether a sequence of events can _actually_ happen that causes a stale pointer dereference. But I don't want to take the chance!)