fix (normal, narrow) printf erroneously processing %n after output errors

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.

src/stdio/vfprintf.c

@@ -530,6 +530,9 @@ static int printf_core(FILE *f, const char *fmt, va_list *ap, union arg *nl_arg,
 
 		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;