drm/nouveau/mem: guard against NULL pointer access in mem_del

other drivers seems to do something similar

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Cc: dri-devel <dri-devel@lists.freedesktop.org>
Cc: Dave Airlie <airlied@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201006220528.13925-2-kherbst@redhat.com