drm/nouveau/fbcon: fix NULL pointer access in nouveau_fbcon_destroy

When the fbcon object is initialized, but nouveau_fbcon_create is not
called, we run into a NULL pointer access within nouveau_fbcon_create when
unloading nouveau.

The call to drm_fb_helper_funcs.fb_probe is deferred until there is a
display for real since 4.14, that's why fbcon->helper.fb is still not set.
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_fbcon.c

@@ -429,7 +429,7 @@ nouveau_fbcon_destroy(struct drm_device *dev, struct nouveau_fbdev *fbcon)
 	drm_fb_helper_unregister_fbi(&fbcon->helper);
 	drm_fb_helper_fini(&fbcon->helper);
 
-	if (nouveau_fb->nvbo) {
+	if (nouveau_fb && nouveau_fb->nvbo) {
 		nouveau_vma_del(&nouveau_fb->vma);
 		nouveau_bo_unmap(nouveau_fb->nvbo);
 		nouveau_bo_unpin(nouveau_fb->nvbo);