drm/virtio: Correct drm_gem_shmem_get_sg_table() error handling

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: c24968734abf ("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.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>