message: Fix uint8_t -> char implicit conversion (UBSan warning)
diff: @@ -173,7 +173,8 @@ parser_result<std::string> parse_string(const uint8_t* pos, const uint8_t* end)
  if (!utf8_validate(pos, pos + size))
  throw parser_error{"Invalid UTF-8"};
  - auto ret = std::string(pos, pos + size);
  + const auto char_begin = reinterpret_cast<const char*>(pos);
  + auto ret = std::string(char_begin, char_begin + size);
  pos += size;
  return {std::move(ret), pos};
  