message: Fix possible crash in `actor` hash function
  The crash occurred when passing a default constructed actor handle to
  the hash function.
diff: @@ -232,7 +232,7 @@ namespace std {
  template <>
  struct hash<caf::actor> {
  inline size_t operator()(const caf::actor& ref) const {
  - return static_cast<size_t>(ref->id());
  + return static_cast<size_t>(ref ? ref->id() : 0);
  }
  };
  } // namespace std
  