unlike with wide printf variants, encoding errors are not a vector by
which this bug is reachable, and the out() helper function already
ensured that no further output could be written after an output error,
transient or otherwise. however, the %n specifier could still be
processed after an error, yielding a side effect that wrongly implied
output had succeeded.
due to buffering effects, it's still possible for %n to show output as
having "succeeded", but for it never to appear on the underlying file
due to an error at flush time. this change, however, ensures that
processing of %n does not conflict with any error which has already
been seen.
if (!f) continue;
+ /* Do not process any new directives once in error state. */
+ if (ferror(f)) return -1;
+
z = buf + sizeof(buf);
prefix = "-+ 0X0x";
pl = 0;