d3d9: Properly retrieve an interface pointer in IDirect3DDevice9Impl_GetBackBuffer().

dlls/d3d9/device.c

@@ -596,6 +596,7 @@ static HRESULT WINAPI IDirect3DDevice9Impl_GetBackBuffer(IDirect3DDevice9Ex *ifa
 {
     IDirect3DDevice9Impl *This = impl_from_IDirect3DDevice9Ex(iface);
     struct wined3d_surface *wined3d_surface = NULL;
+    IDirect3DSurface9Impl *surface_impl;
     HRESULT hr;
 
     TRACE("iface %p, swapchain %u, backbuffer_idx %u, backbuffer_type %#x, backbuffer %p.\n",
@@ -606,7 +607,8 @@ static HRESULT WINAPI IDirect3DDevice9Impl_GetBackBuffer(IDirect3DDevice9Ex *ifa
             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->IDirect3DSurface9_iface;
         IDirect3DSurface9_AddRef(*ppBackBuffer);
         wined3d_surface_decref(wined3d_surface);
     }