fuse_lowlevel: Free mountpoint in fuse_session_destroy

If fuse_session_mount() succeeds but fuse_session_unmount() is never
called (e.g., on an error path), the mountpoint string allocated by
strdup in fuse_session_mount() is leaked. Free it in
fuse_session_destroy() using the same atomic_exchange pattern as
fuse_session_unmount() to handle concurrent access safely.

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

lib/fuse_lowlevel.c

@@ -4163,6 +4163,7 @@ void fuse_session_destroy(struct fuse_session *se)
 		pthread_mutex_unlock(&se->timeout_thread->lock);
 	}
 
+	free(atomic_exchange(&se->mountpoint, NULL));
 	free(se);
 }