Find out if image is 100% black

I am trying to find out if the screenshot I take is 100% black before I save it. If yes then show an error. My code is:

#include "stdafx.h"
#include <windows.h>
#include <GdiPlus.h>
#include <string>

using namespace std;
#pragma comment( lib, "gdiplus" )
int GetEncoderClsid(const WCHAR* format, CLSID* pClsid);

void gdiscreen(string save_path_s)
{
	using namespace Gdiplus;
	GdiplusStartupInput gdiplusStartupInput;
	ULONG_PTR gdiplusToken;
	GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);
	
	{
		HDC scrdc, memdc;
		HBITMAP membit;
		scrdc = ::GetDC(0);
		int Height = GetSystemMetrics(SM_CYSCREEN);
		int Width = GetSystemMetrics(SM_CXSCREEN);
		memdc = CreateCompatibleDC(scrdc);
		membit = CreateCompatibleBitmap(scrdc, Width, Height);
		HBITMAP hOldBitmap =(HBITMAP) SelectObject(memdc, membit);
		BitBlt(memdc, 0, 0, Width, Height, scrdc, 0, 0, SRCCOPY);

		Gdiplus::Bitmap bitmap(membit, NULL);
                //I have no idea but the solution must be something with bitmap/membit.
		CLSID clsid;
		GetEncoderClsid(L"image/jpeg", &clsid);
		
		wstring save_path(save_path_s.length(), L' ');
		copy(save_path_s.begin(), save_path_s.end(), save_path.begin());

		bitmap.Save(save_path.c_str(), &clsid);

		SelectObject(memdc, hOldBitmap);

		DeleteObject(memdc);

		DeleteObject(membit);

		::ReleaseDC(0,scrdc);
	}

	GdiplusShutdown(gdiplusToken);
}

int GetEncoderClsid(const WCHAR* format, CLSID* pClsid)
{
	using namespace Gdiplus;
	UINT  num = 0;          // number of image encoders
	UINT  size = 0;         // size of the image encoder array in bytes

	ImageCodecInfo* pImageCodecInfo = NULL;

	GetImageEncodersSize(&num, &size);
	if(size == 0)
		return -1;  // Failure

	pImageCodecInfo = (ImageCodecInfo*)(malloc(size));
	if(pImageCodecInfo == NULL)
		return -1;  // Failure

	GetImageEncoders(num, size, pImageCodecInfo);

	for(UINT j = 0; j < num; ++j)
	{
		if( wcscmp(pImageCodecInfo[j].MimeType, format) == 0 )
		{
			*pClsid = pImageCodecInfo[j].Clsid;
			free(pImageCodecInfo);
			return j;  // Success
		}    
	}

	free(pImageCodecInfo);
	return 0;
}

void main()
{
	gdiscreen("C:\\screenshot.jpg");
}

Pls help me solve this!