message: Fix char to int conversion error
  This problem in query parser would cause a crash on Visual Studio in
  Debug builds. Credits to simonask for finding this.
diff: @@ -61,7 +61,9 @@ std::string print_value<>(realm::null)
  bool contains_invalids(StringData data) {
  const static std::string whitelist = " {|}~:;<=>?@!#$%&()*+,-./[]^_`";
  for (size_t i = 0; i < data.size(); ++i) {
  - if (!std::isalnum(data.data()[i]) && whitelist.find(data.data()[i]) == std::string::npos) {
  + using unsigned_char_t = unsigned char;
  + char c = data.data()[i];
  + if (!std::isalnum(unsigned_char_t(c)) && whitelist.find(c) == std::string::npos) {
  return true;
  }
  }
  