sandbox: Double free when initializing the HSv3 config

In main.c:

       /* steals references */
       sandbox_cfg_allow_open_filename(&cfg, file_name);
       sandbox_cfg_allow_open_filename(&cfg, tmp_name);
       tor_free(file_name);

... freeing after stealing a reference is really not good :).

Introduced in 5d2506d70cd which was never released.