d3d8: Properly retrieve an interface pointer in IDirect3DCubeTexture8Impl_UnlockRect().

dlls/d3d8/cubetexture.c

@@ -338,6 +338,7 @@ static HRESULT WINAPI IDirect3DCubeTexture8Impl_UnlockRect(IDirect3DCubeTexture8
 {
     IDirect3DCubeTexture8Impl *texture = impl_from_IDirect3DCubeTexture8(iface);
     struct wined3d_resource *sub_resource;
+    IDirect3DSurface8Impl *surface_impl;
     UINT sub_resource_idx;
     HRESULT hr;
 
@@ -348,7 +349,10 @@ static HRESULT WINAPI IDirect3DCubeTexture8Impl_UnlockRect(IDirect3DCubeTexture8
     if (!(sub_resource = wined3d_texture_get_sub_resource(texture->wined3d_texture, sub_resource_idx)))
         hr = D3DERR_INVALIDCALL;
     else
-        hr = IDirect3DSurface8_UnlockRect((IDirect3DSurface8 *)wined3d_resource_get_parent(sub_resource));
+    {
+        surface_impl = wined3d_resource_get_parent(sub_resource);
+        hr = IDirect3DSurface8_UnlockRect(&surface_impl->IDirect3DSurface8_iface);
+    }
     wined3d_mutex_unlock();
 
     return hr;