message: Change usage of std::string's .at() to more widely-compatible [].
diff: @@ -1327,7 +1327,7 @@ CheckedError Parser::TryTypedValue(const std::string *name, int dtoken,
  const auto &s = e.constant;
  const auto k = s.find_first_of("0123456789.");
  if ((std::string::npos != k) && (s.length() > (k + 1)) &&
  - (s.at(k) == '0' && is_alpha_char(s.at(k + 1), 'X')) &&
  + (s[k] == '0' && is_alpha_char(s[k + 1], 'X')) &&
  (std::string::npos == s.find_first_of("pP", k + 2))) {
  return Error(
  "invalid number, the exponent suffix of hexadecimal "
  