fuse_uring: Null pool pointer after fuse_uring_stop

fuse_uring_stop() frees the ring pool via fuse_session_destruct_uring()
but does not null se->uring.pool afterwards, leaving a dangling pointer.
While no current code path dereferences the pool after stop, this is a
latent maintenance hazard — any future access would hit freed memory.

The error path in fuse_uring_start() already nulls se->uring.pool on
failure; apply the same discipline in the normal teardown path.

Signed-off-by: Abhinav Agarwal <abhinavagarwal1996@gmail.com>

lib/fuse_uring.c

@@ -932,6 +932,7 @@ int fuse_uring_stop(struct fuse_session *se)
 		return 0;
 
 	fuse_session_destruct_uring(ring);
+	se->uring.pool = NULL;
 
 	return 0;
 }