libkiconv: address memory leak in not-found cases

Found in "Understanding and Detecting Disordered Error Handling with
Precise Function Pairing" by Qiushi Wu et al.

Reviewed by: imp, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29896

(cherry picked from commit 3cfd08c1c74058451a02bac35307bf7fa509c617)