libiconv VIQR: Fix a use after free.

Use TAILQ_FOREACH_SAFE to walk to list of children mnemonics to free
them instead of TAILQ_FOREACH.

Reviewed by: emaste
Reported by: GCC 12 -Wuse-after-free
Differential Revision: https://reviews.freebsd.org/D36821

(cherry picked from commit 8f27c9d14a559f33aa7fc3245f841f7ce52fadd5)