fold: fix write error checks with invalid multi-byte input

fold: fix write error checks with invalid multi-byte input

* src/fold.c (write_out): A new helper to check all writes.
(fold-file): Use write_out() for all writes.
* tests/fold/fold-zero-width.sh: Adjust to writing more
data in various patterns, rather than two buffers of NULs.
This is a more robust memory bound check, and the '\303' case
tests this particular logic change.
* NEWS: fold now exits immediately, not just promptly.