message: Fix segfault when setting an invalid credit policy
  Fixes
diff: @@ -78,9 +78,11 @@ public:
  controller_ = detail::token_based_credit_controller::make(self(), in);
  else if (*str == "size-based")
  set_default();
  - else
  + else {
  + set_default();
  CAF_LOG_WARNING("unrecognized credit policy:"
  << *str << "(falling back to 'size-based')");
  + }
  } else {
  set_default();
  }
  