observe callback must be a no-op after cancellation

[lgrapenthin]

example from #4 in cljs

Unlike the CLJ version, this version yields a result. I. e. it prints

res [0 1 2]

But shortly after that, an uncaught type error is thrown in impl.cljs

(defn process-producer [event-fn]
  (let [step
        (fn step [i]
          (if (= 5 i)
            nil
            (js/setTimeout (fn []
                             (event-fn i)
                             (step (inc i)))
                           1000)))]
    (step 0)
    (fn []
      (.log js/console "Process producer cancelled"))))

(defn printer [t]
  (t (fn [res] (println "res" res))
     (fn [err] (println "err" err))))

(def inst
  (printer (->> (m/observe
                 process-producer)
                (m/transform (take 3))
                (m/aggregate conj))))

[leonoel]

Bug. It should not really happen because unsubscription is supposed to prevent the event callback from being called again, but if it happens anyhow the callback should return normally.