free_prompts: deal with a reference from an Ldisc.

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!)

utils/prompts.c

@@ -17,6 +17,7 @@ prompts_t *new_prompts(void)
     p->name_reqd = p->instr_reqd = false;
     p->callback = NULL;
     p->callback_ctx = NULL;
+    p->ldisc_ptr_to_us = NULL;
     return p;
 }
 
@@ -49,6 +50,12 @@ char *prompt_get_result(prompt_t *pr)
 void free_prompts(prompts_t *p)
 {
     size_t i;
+
+    /* If an Ldisc currently knows about us, tell it to forget us, so
+     * it won't dereference a stale pointer later. */
+    if (p->ldisc_ptr_to_us)
+        *p->ldisc_ptr_to_us = NULL;
+
     for (i=0; i < p->n_prompts; i++) {
         prompt_t *pr = p->prompts[i];
         strbuf_free(pr->result);