Creation of a subprocess create an error Access violation writing location during execution

I would like to execute several cmd commands in C++ with the same shell. I precise the same shell because I would like to execute for example a command cd .. and dir or something else. I found this code on internet but I can't use it because when I launch it, it prompt an error : Exception thrown at 0x67B21010 (KernelBase.dll) in Shell.exe: 0xC0000005: Access violation writing location 0x007C6152.

What I am doing wrong ? This is the source code :

#include <iostream>
#include <atlstr.h>

CStringA shell(const wchar_t* cmd)
{
    CStringA result;
    HANDLE hPipeRead, hPipeWrite;

    SECURITY_ATTRIBUTES saAttr = { sizeof(SECURITY_ATTRIBUTES) };
    saAttr.bInheritHandle = TRUE;
    saAttr.lpSecurityDescriptor = NULL;


    if (!CreatePipe(&hPipeRead, &hPipeWrite, &saAttr, 0))
        return result;

    STARTUPINFOW si = { sizeof(STARTUPINFOW) };
    si.dwFlags = STARTF_USESHOWWINDOW | STARTF_USESTDHANDLES;
    si.hStdOutput = hPipeWrite;
    si.hStdError = hPipeWrite;
    si.wShowWindow = SW_HIDE;

    PROCESS_INFORMATION pi = { 0 };

    BOOL fSuccess = CreateProcessW(NULL, (LPWSTR)cmd, NULL, NULL, TRUE, CREATE_NEW_CONSOLE, NULL, NULL, &si, &pi);
    if (!fSuccess)
    {
        CloseHandle(hPipeWrite);
        CloseHandle(hPipeRead);
        return result;
    }

    bool bProcessEnded = false;
    for (; !bProcessEnded;)
    {
        bProcessEnded = WaitForSingleObject(pi.hProcess, 50) == WAIT_OBJECT_0;

        for (;;)
        {
            char buf[1024];
            DWORD dwRead = 0;
            DWORD dwAvail = 0;

            if (!::PeekNamedPipe(hPipeRead, NULL, 0, NULL, &dwAvail, NULL))
                break;

            if (!dwAvail)
                break;

            if (!::ReadFile(hPipeRead, buf, min(sizeof(buf) - 1, dwAvail), &dwRead, NULL) || !dwRead)
                break;

            buf[dwRead] = 0;
            result += buf;
        }
    }

    CloseHandle(hPipeWrite);
    CloseHandle(hPipeRead);
    CloseHandle(pi.hProcess);
    CloseHandle(pi.hThread);
    return result;
}

int main()
{
    std::cout << "Hello World!\n";
    std::wcout << shell(L"cmd.exe") << std::endl;
    std::wcout << shell(L"cd ..") << std::endl;
    std::wcout << shell(L"dir") << std::endl;
}