glShaderSource access violation when using Win32 to read files

So, I'm trying to setup basic OpenGL scene using only Win32 API and OpenGL, but I've big problems with loading shaders and glShaderSource function. I'm reading my file like this:

//HEADER

class FileReader
{
public:
    FileReader(const LPCSTR FileName);

    const void* GetFileData();

    ~FileReader();
private:

    HANDLE FileHandle;

    void* FileDataMemory;
};

//ACTUAL CODE

FileReader::FileReader(const LPCSTR FileName)
{
    FileHandle = CreateFileA(FileName, GENERIC_READ, FILE_SHARE_READ, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
    if (FileHandle == INVALID_HANDLE_VALUE)
    {
        OutputDebugStringA("Failed to open file ");
        OutputDebugStringA(FileName);
        OutputDebugStringA(" for reading, application could not be loaded\n");
        ExitProcess(-2);
    }

    unsigned int FileSize = GetFileSize(FileHandle, 0);
    DWORD BytesRead;
    FileDataMemory = VirtualAlloc(0, FileSize, MEM_COMMIT, PAGE_READWRITE);
    ReadFile(FileHandle, FileDataMemory, FileSize, &BytesRead, 0);
    if (BytesRead < FileSize)
    {
        OutputDebugStringA("File was not read completely\n");
    }
}

const void* FileReader::GetFileData()
{
    return FileDataMemory;
}

FileReader::~FileReader()
{
    CloseHandle(FileHandle);
}

And I use this class to load vertex shader from disc like this:

    VertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(VertexShader, 1, static_cast<const GLchar* const*>(VertexShaderFile->GetFileData()), 0);

But my app gives me an access violation reading address 0xFFFFFFFF on line with glShaderSource. And I'm just so confused, because when I try to see this block of memory in debug mode, it looks properly and has correct data insisde it, so I just don't know what to do.

The shader source code string, which is read form the file is not 0 terminated.
Reserve FileSize+1 bytes of memory and ensure that the last byte of the buffer is 0. e.g.:

FileDataMemory = VirtualAlloc(0, FileSize+1, MEM_COMMIT, PAGE_READWRITE);
ZeroMemory(FileDataMemory, FileSize+1);
ReadFile(FileHandle, FileDataMemory, FileSize, &BytesRead, 0);

Further, the 3rd parameter to glShaderSource is an array of strings (const GLchar **):

const GLchar *source = static_cast<const GLchar*>(VertexShaderFile->GetFileData());
glShaderSource(VertexShader, 1, &source, 0);

What you actually do in your code is to cast VertexShaderFile->GetFileData(). What you would have to do ist to cast &VertexShaderFile->GetFileData()

Furthermore, I recommend to use STL to read the shader source file. e.g:

std::ifstream sourceFile(FileName, std::fstream::in); 
std::string sourceCode = std::string( 
    std::istreambuf_iterator<char>(sourceFile),
    std::istreambuf_iterator<char>());