d3d8: Properly retrieve an interface pointer in IDirect3DSwapChain8Impl_GetBackBuffer().

dlls/d3d8/swapchain.c

@@ -110,6 +110,7 @@ static HRESULT WINAPI IDirect3DSwapChain8Impl_GetBackBuffer(IDirect3DSwapChain8
 {
     IDirect3DSwapChain8Impl *This = impl_from_IDirect3DSwapChain8(iface);
     struct wined3d_surface *wined3d_surface = NULL;
+    IDirect3DSurface8Impl *surface_impl;
     HRESULT hr;
 
     TRACE("iface %p, backbuffer_idx %u, backbuffer_type %#x, backbuffer %p.\n",
@@ -120,7 +121,8 @@ static HRESULT WINAPI IDirect3DSwapChain8Impl_GetBackBuffer(IDirect3DSwapChain8
             iBackBuffer, (enum wined3d_backbuffer_type)Type, &wined3d_surface);
     if (SUCCEEDED(hr) && wined3d_surface)
     {
-        *ppBackBuffer = wined3d_surface_get_parent(wined3d_surface);
+        surface_impl = wined3d_surface_get_parent(wined3d_surface);
+        *ppBackBuffer = &surface_impl->IDirect3DSurface8_iface;
         IDirect3DSurface8_AddRef(*ppBackBuffer);
         wined3d_surface_decref(wined3d_surface);
     }