drm/virtio: Correct drm_gem_shmem_get_sg_table() error handling

Previous commit fixed checking of the ERR_PTR value returned by
drm_gem_shmem_get_sg_table(), but it missed to zero out the shmem->pages,
which will crash virtio_gpu_cleanup_object(). Add the missing zeroing of
the shmem->pages.

Fixes: c2496873 ("drm/virtio: Fix NULL vs IS_ERR checking in virtio_gpu_object_shmem_init")
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20220630200726.1884320-2-dmitry.osipenko@collabora.comSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/virtgpu_object.c

@@ -170,6 +170,7 @@ static int virtio_gpu_object_shmem_init(struct virtio_gpu_device *vgdev,
 	shmem->pages = drm_gem_shmem_get_sg_table(&bo->base);
 	if (IS_ERR(shmem->pages)) {
 		drm_gem_shmem_unpin(&bo->base);
+		shmem->pages = NULL;
 		return PTR_ERR(shmem->pages);
 	}