[GEN][ZH] Fix use of invalid back-buffer surface pointer

By checking the result of GetBackBuffer,
passing an invalid (usually uninitialized) pointer is avoided.

Author: Fighter19

Generals/Code/Libraries/Source/WWVegas/WW3D2/dx8wrapper.cpp

@@ -2405,8 +2405,9 @@ SurfaceClass * DX8Wrapper::_Get_DX8_Back_Buffer(unsigned int num)
 
 	IDirect3DSurface8 * bb;
 	SurfaceClass *surf=NULL;
-	DX8CALL(GetBackBuffer(num,D3DBACKBUFFER_TYPE_MONO,&bb));
-	if (bb)
+	HRESULT hres=S_FALSE;
+	DX8CALL_HRES(GetBackBuffer(num,D3DBACKBUFFER_TYPE_MONO,&bb), hres);
+	if (hres==D3D_OK)
 	{
 		surf=NEW_REF(SurfaceClass,(bb));
 		bb->Release();

GeneralsMD/Code/Libraries/Source/WWVegas/WW3D2/dx8wrapper.cpp

@@ -3163,8 +3163,9 @@ SurfaceClass * DX8Wrapper::_Get_DX8_Back_Buffer(unsigned int num)
 
 	IDirect3DSurface8 * bb;
 	SurfaceClass *surf=NULL;
-	DX8CALL(GetBackBuffer(num,D3DBACKBUFFER_TYPE_MONO,&bb));
-	if (bb)
+	HRESULT hres=S_FALSE;
+	DX8CALL_HRES(GetBackBuffer(num,D3DBACKBUFFER_TYPE_MONO,&bb), hres);
+	if (hres==D3D_OK)
 	{
 		surf=NEW_REF(SurfaceClass,(bb));
 		bb->Release();