makefs: Quiet a -Wdangling-pointer warning in cd9660_generate_path_table

This function temporarily stores a pointer to an on-stack variable (a
TAILQ_HEAD of a temporary list) into a global variable (*n).

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D42581

(cherry picked from commit 5fc8ed9313609fbafe505deaea253e6bde5ac165)