drm/nouveau: allow nouveau_fence_ref() to be a noop

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

drivers/gpu/drm/nouveau/nouveau_fence.c

@@ -306,7 +306,8 @@ nouveau_fence_unref(struct nouveau_fence **pfence)
 struct nouveau_fence *
 nouveau_fence_ref(struct nouveau_fence *fence)
 {
-	kref_get(&fence->kref);
+	if (fence)
+		kref_get(&fence->kref);
 	return fence;
 }