Troublesome 2D array realloc heap error - c

Question

Code snippet follows.
Background: Revising a old C windows application to communicate with new instruments (while maintaining as much old code as possible), which requires the addition of new features. In this case I am copying file names from a directory into a 2D array to populate a list of the file names.
While debugging, the program gets to the realloc call (the second realloc, as the first is skipped by the if statement) and then breaks with the error which precedes the code snippet below.
At the time of realloc, WFMLIST has been allocated, with the first element initialized to 0, and NbItemWFMLIST = 1. So my question is, does anybody know why this error might be occurring? The memory allocation and reallocation worked before I added the directory searching and copying components. Is it DLL releated? I have tried many things (suggestions from similar errors on Stackoverflow), including replacing the realloc with a free() and then calloc() and received the same error.
Also, ffd is correctly finding the directory and providing the appropriate string in ffd.cFileName.
Please let me know if I can provide more information. Thanks.
HEAP[vhf.exe]: Heap block at 01068980 modified at 01068989 past
requested size of 1 Windows has triggered a breakpoint in vhf.exe.
This may be due to a corruption of the heap, which indicates a bug in
vhf.exe or any of the DLLs it has loaded.
This may also be due to the user pressing F12 while vhf.exe has focus.
char **WMFLIST=NULL;
long int NbItemWFMLIST = 0;
unsigned int playback_bytes;
WIN32_FIND_DATA ffd; //Added for directory search
LARGE_INTEGER filesize; //
TCHAR szDir[MAX_PATH]; //
TCHAR *pwrkDir;
size_t length_pwrkDir;
HANDLE hFind = INVALID_HANDLE_VALUE;
pwrkDir = (TCHAR *)calloc(5, sizeof(TCHAR));
if(WFMLIST==NULL)
{
WFMLIST=(char **)calloc(NbItemWFMLIST,sizeof(char *));
if(WFMLIST != NULL)
{
WFMLIST[0]=(char *)calloc(13,sizeof(char));
if(WFMLIST[0]==NULL)
{
MessageBox(hWnd,"CALLOC WMLIST[0]==NULL","Memory allocation error",MB_OK);
NbItemWFMLIST=0;
return(0); **error check
}
else
{
NbItemWFMLIST=1;
sprintf_s(&WFMLIST[NbItemWFMLIST-1][0],MAX_PATH,"\0");
}
}
else
{
**error
}
}
else
{
WFMLIST = (char **)realloc((void *) WFMLIST,sizeof(char *));
if(WFMLIST==NULL)
{
**error check
}
else
{
NbItemWFMLIST=1;
sprintf(&WFMLIST[NbItemWFMLIST-1][0],"\0");
}
}
if((tawg->status = GetCurrentDirectory(MAX_PATH,pwrkDir))==SIG_SUCCESS) //Locate the current working directory
{
**Error check
}
sprintf_s(pwrkDir, MAX_PATH, "%s\\Waveforms", pwrkDir); //Append directory with Waveform folder
strcpy(szDir, pwrkDir);
strcat(szDir,"\\*");
hFind = FindFirstFile(szDir, &ffd);
if (INVALID_HANDLE_VALUE == hFind)
{
**error check
}
while (FindNextFile(hFind, &ffd) != 0);
{
if (ffd.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY)
{
**error check
}
else
{
WFMLIST=(char **)realloc((void*)WFMLIST,((NbItemWFMLIST+1)*sizeof(char *)));
if (WFMLIST[NbItemWFMLIST]==NULL)
{
**error
}
else
{
WFMLIST[NbItemWFMLIST]=(char *)calloc(13,sizeof(char));
if (WFMLIST[NbItemWFMLIST]==NULL)
{
**error
}
else
{
sprintf(&WFMLIST[NbItemWFMLIST][0],"%s",ffd.cFileName);
NbItemWFMLIST=NbItemWFMLIST+1;
}
}
}
}