fix 'Cannot set properties of undefined'

[mjmeintjes]

Please review as I have no understanding of the code, but there was an obvious error in that the pr was undefined (it evaluated to cljs.core.pr, so hiding the problem), so I just replaced it with curr which seems to have fixed the error.

Fixes #77

[mjmeintjes]

After applying this change, the following code throws an exception when cancelling:

Running the following code in CLJS throws an exception while cancelling.

  (do
    (def input! (atom 10))
    (def task
      (ms/reduce
       (constantly nil)
       (ms/ap
        (let [n (or (ms/?< (ms/watch input!))
                    1)
              nxt (ms/?> (ms/seed (range n)))]
          (println nxt)))))
    (def cancel (task println #(js/console.error %)))
    (cancel))

Exception:

Cannot read properties of null (reading 'next')
    at Object.missionary$impl$Ambiguous$walk [as walk] (eval at shadow$cljs$devtools$client$browser$global_eval (shadow.cljs.devtools.client.browser.js:1:1), <anonymous>:5:58)
    at Object.missionary$impl$Ambiguous$ready [as ready] (Ambiguous.cljs:312:49)
    at G__261264.G__261264__0 [as cljs$core$IFn$_invoke$arity$0] (Ambiguous.cljs:323:28)
    at missionary$impl$Watch$kill (Watch.cljs:22:32)
    at Object.eval [as cljs$core$IFn$_invoke$arity$0] (Watch.cljs:7:20)
    at missionary$impl$Ambiguous$cancel (Ambiguous.cljs:92:9)
    at missionary$impl$Ambiguous$walk (eval at shadow$cljs$devtools$client$browser$global_eval (shadow.cljs.devtools.client.browser.js:1:1), <anonymous>:5:34)
    at missionary$impl$Ambiguous$kill (Ambiguous.cljs:85:6)

[leonoel]

You're right, it's a typo. pr was supposed to be an alias for curr so it's safe to just refer to curr.
Your fix is incomplete though, pr must be replaced with curr on L304-305 too. You can refer to the java implementation for reference

missionary/java/missionary/impl/Ambiguous.java

Lines 387 to 392 in 0e08527

	Processor pr = (Processor) curr;
	Branch pivot = pr.child;
	b.current = pivot;
	pivot.parent = b;
	while ((pr = pr.next) != curr) move(pivot, pr.child);
	pr.prev = pr.next = pr;