od(1): Fix wrong output for some corner cases in multibyte locales.

Restore the original character to print if we used the look-ahead
buffer, but that didn't help -- we either got an illegal sequence
or still can't complete.

PR: 224552
Submitted by: Yuri Pankov
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D13963