d3d8: Properly retrieve an interface pointer in IDirect3DDevice8Impl_GetBackBuffer().

dlls/d3d8/device.c

@@ -619,6 +619,7 @@ static HRESULT WINAPI IDirect3DDevice8Impl_GetBackBuffer(IDirect3DDevice8 *iface
 {
     IDirect3DDevice8Impl *This = impl_from_IDirect3DDevice8(iface);
     struct wined3d_surface *wined3d_surface = NULL;
+    IDirect3DSurface8Impl *surface_impl;
     HRESULT hr;
 
     TRACE("iface %p, backbuffer_idx %u, backbuffer_type %#x, backbuffer %p.\n",
@@ -629,7 +630,8 @@ static HRESULT WINAPI IDirect3DDevice8Impl_GetBackBuffer(IDirect3DDevice8 *iface
             BackBuffer, (enum wined3d_backbuffer_type)Type, &wined3d_surface);
     if (SUCCEEDED(hr) && wined3d_surface && ppBackBuffer)
     {
-        *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);
     }