关于《Windows程序设计（第五版）》中一个实例程序的疑问

最近一直在看Charlse Petzold的《Windows程序设计》，作为一个新得不能再新的新手，只能先照着书的抄抄源码了，之前的例子一直都很正常，但昨天遇到一个很诡异的BUG。

先看实例源码吧：

 /*-----------------------------------------------------------------
         ENVIRON.C -- Environment List Box
         (c) Charles Petzold,1998
         Copy by XXXX,2015/1/26
 ------------------------------------------------------------------*/
 
 #include <windows.h>
 
 #define ID_LIST 1
 #define ID_TEXT 2
 
 LRESULT CALLBACK WndProc(HWND hwnd, UINT, WPARAM, LPARAM);
 
 int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int iCmdShow)
 {
     static TCHAR szAppName[] = TEXT("Environ");
     HWND hwnd;
     MSG msg;
     WNDCLASS wndclass;
 
     wndclass.style = CS_VREDRAW | CS_HREDRAW;
     wndclass.lpfnWndProc = WndProc;
     wndclass.cbClsExtra = ;
     wndclass.cbWndExtra = ;
     wndclass.hInstance = hInstance;
     wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
     wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
     wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
     wndclass.lpszMenuName = NULL;
     wndclass.lpszClassName = szAppName;
 
     if (!RegisterClass(&wndclass))
     {
         MessageBox(NULL, TEXT("This program requires Windows NT!"), szAppName, MB_ICONERROR);
         return ;
     }
 
     hwnd = CreateWindow(szAppName, TEXT("Environment List Box"), WS_OVERLAPPEDWINDOW,
         CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL, NULL, hInstance, NULL);
     ShowWindow(hwnd, iCmdShow);
     UpdateWindow(hwnd);
 
     while (GetMessage(&msg, NULL, , ))
     {
         TranslateMessage(&msg);
         DispatchMessage(&msg);
     }
     return msg.wParam;
 }
 
 void FillListBox(HWND hwndList)
 {
     int iLength;
     TCHAR * pVarBlock, *pVarBeg, *pVarEnd, *pVarName;
     pVarBlock = GetEnvironmentStrings();    //Get pointer to environment block
 
     while (*pVarBlock)
     {
         if (*pVarBlock != '=')    //Skip variable names begining with '='
         {
             pVarBeg = pVarBlock;    //Beging of variable name
             while (*(pVarBlock++) != '=');    //Scan until '='
             pVarEnd = pVarBlock - ;    //Points to '=' sign
             iLength = pVarEnd - pVarBeg;    //Length of variable name
 
             //Allocate memory for the variable name and terminating
             //zero.Copy the variable name and append a zero
             pVarName = calloc(iLength + , sizeof(TCHAR));
             CopyMemory(pVarName, pVarBeg, iLength * sizeof(TCHAR));
             pVarName[iLength] = '\0';
 
             //Put the variable name int the list box and free memory
             SendMessage(hwndList, LB_ADDSTRING, , (LPARAM)pVarName);
             free(pVarName);
         }
         while (*pVarBlock++ != '\0');    //Scna until terminating zero
 
     }
     FreeEnvironmentStrings(pVarBlock);
     int code;
     code = GetLastError();
 }
 
 LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
 {
     static HWND hwndList, hwndText;
     int iIndex, iLength, cxChar, cyChar;
     TCHAR * pVarName, *pVarValue;
 
     switch (message)
     {
     case WM_CREATE:
         cxChar = LOWORD(GetDialogBaseUnits());
         cyChar = HIWORD(GetDialogBaseUnits());
 
         //Create listbox and static text windows
         hwndList = CreateWindow(TEXT("listbox"), NULL, WS_CHILD | WS_VISIBLE | LBS_STANDARD,
             cxChar, cyChar * , cxChar *  + GetSystemMetrics(SM_CXVSCROLL), cyChar * , hwnd,
             (HMENU)ID_LIST, (HINSTANCE)GetWindowLong(hwnd, GWL_HINSTANCE), NULL);
         hwndText = CreateWindow(TEXT("static"), NULL, WS_CHILD | WS_VISIBLE | SS_LEFT, cxChar, cyChar,
             GetSystemMetrics(SM_CXSCREEN), cyChar, hwnd, (HMENU)ID_TEXT,
             (HINSTANCE)GetWindowLong(hwnd, GWL_HINSTANCE), NULL);
         FillListBox(hwndList);
         return ;
 
     case WM_SETFOCUS:
         SetFocus(hwndList);
         return ;
 
     case WM_COMMAND:
         if (LOWORD(wParam) == ID_LIST&&HIWORD(wParam) == LBN_SELCHANGE)
         {
             //Get current selectiong
             iIndex = SendMessage(hwndList, LB_GETCURSEL, , );
             iLength = SendMessage(hwndList, LB_GETTEXTLEN, iIndex, ) + ;
             pVarName = calloc(iLength, sizeof(TCHAR));
             SendMessage(hwndList, LB_GETTEXT, iIndex, (LPARAM)pVarName);
 
             //Get environment string
             iLength = GetEnvironmentVariable(pVarName, NULL, );
             pVarValue = calloc(iLength, sizeof(TCHAR));
             GetEnvironmentVariable(pVarName, pVarValue, iLength);
 
             //Show it in window
             SetWindowText(hwndText, pVarValue);
             free(pVarName);
             free(pVarValue);
         }
         return ;
 
     case WM_DESTROY:
         PostQuitMessage();
         return ;
     }
     return DefWindowProc(hwnd, message, wParam, lParam);
 }

编译什么的都没有错误提示，但是在本地调试时提示触发一个断点，但是我确定我没有下过断点！！！

提示如图：

选择“继续”后会又提示一个中断，提示堆被损坏

作为一个小彩笔，真心弄不懂，求教大牛！！！