message: Improve assertions in LIFO inbox
diff: @@ -123,9 +123,13 @@ public:
  // empty.
  CAF_ASSERT(new_head == stack_closed_tag()
  || new_head == stack_empty_tag());
  - // Must not be called on a closed queue.
  pointer e = stack_.load();
  + // Must not be called on a closed queue.
  CAF_ASSERT(e != stack_closed_tag());
  + // Must not be called on a blocked queue unless for setting it to closed,
  + // because that would mean an actor accesses its mailbox after blocking its
  + // mailbox but before receiving anything.
  + CAF_ASSERT(e != reader_blocked_tag() || new_head == stack_closed_tag());
  // We don't assert these conditions again since only the owner is allowed
  // to call this member function, i.e., there's never a race on `take_head`.
  while (e != new_head) {
  