fix: using stream operator not set eofbit

[Night12138]

In the current version, there are the following issue

#include <iostream>
#include "picojson.hpp"

int main()
{
    while (!std::cin.eof()) // won't break
    {
        picojson::value v;
        std::cin >> v;
        // processing
        const auto &err = picojson::get_last_error();
        std::cout << "err: " << err << std::endl;
        std::cout << "res: " << v << std::endl;
    }
    return 0;
}

at Line 6, when read eof state from std::cin always return false.

fix: add beg/end judge in parse(value &out, std::istream &is) at Line 1135

inline std::string parse(value &out, std::istream &is) {
  std::string err;
  const auto &beg = std::istreambuf_iterator<char>(is.rdbuf());
  const auto &end = std::istreambuf_iterator<char>();
  parse(out, beg, end, &err);
  if (beg == end) {
    is.setstate(std::ios_base::eofbit); // set eof state because stream has no more data available
  }
  return err;
}