message: fix sstream::c_str() when the buffer is first created
diff: @@ -25,7 +25,8 @@ utils::io::ostream& sstream::flush() noexcept {
  }
  const char* sstream::c_str() const noexcept {
  - return mData.get();
  + char const* buffer = mData.get();
  + return buffer ? buffer : "";
  }
  } // namespace io
  