message: Fix bug in actor_config's to_string
diff: @@ -31,11 +31,9 @@ actor_config::actor_config(execution_unit* ptr)
  }
  std::string to_string(const actor_config& x) {
  + // Note: x.groups is an input range. Traversing it is emptying it, hence we
  + // cannot look inside the range here.
  std::string result = "actor_config(";
  - if (x.groups == nullptr)
  - result += "[]";
  - else
  - result += deep_to_string(*x.groups);
  auto add = [&](int flag, const char* name) {
  if ((x.flags & flag) != 0) {
  result += ", ";
  