drm/nouveau/nvif: return null pointers on failure, in addition to ret != 0

Reported by Coverity.  The intention is that the return value is
checked, but let's be more paranoid and make it extremely obvious
if something forgets to.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

diff --cc drivers/gpu/drm/nouveau/nvif/notify.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nvif/object.c
Simple merge