nouveau/winsys: Allow subchan_dealloc() on zeroed subchans

This makes cleanup a little more robust.  I'm not sure it's strictly
needed as the kernel will probably just error in that case but it's
probably safer to not call destroy on something that isn't registered or
is already destroyed.

Reviewed-by: Mel Henning <mhenning@darkrefraction.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36641>

src/nouveau/winsys/nouveau_context.c

@@ -11,6 +11,9 @@
 static void
 nouveau_ws_subchan_dealloc(int fd, struct nouveau_ws_object *obj)
 {
+   if (obj->cls == 0)
+      return;
+
    struct {
       struct nvif_ioctl_v0 ioctl;
       struct nvif_ioctl_del del;