MFC r328188,r328189,r328200: Fix wrong output for multibyte corner cases

MFC r328188: od(1): Fix wrong output for 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.

MFC r328189: od(1): Fix mis-patch from r328188

od_test.sh got duplicated erroneously when it was added in r328188. Dedup.

MFC r328200: Silence the gcc warning: 'op' may be used uninitialized in this
function

PR: 224552