message: Fix heap-use-after-free in merge operator
diff: @@ -130,11 +130,13 @@ public:
  void fwd_on_next(input_key key, const observable<T>& item) {
  CAF_LOG_TRACE(CAF_ARG(key) << CAF_ARG(item));
  - if (auto ptr = get(key)) {
  + if (auto ptr = get(key))
  subscribe_to(item);
  + // Note: we need to double-check that the key still exists here, because
  + // subscribe_on may result in an error (that nukes all inputs).
  + if (auto ptr = get(key))
  ptr->sub.request(1);
  }
  - }
  // -- implementation of subscription_impl ------------------------------------
  