make vfprintf set stream orientation even for zero-length output

if no output is produced, no underlying fwrite will ever be called,
but byte-oriented printf functions are still required to set the
orientation of the stream to byte-oriented. call __towrite explicitly
if the FILE is not already in write mode.

src/stdio/vfprintf.c

@@ -678,7 +678,8 @@ int vfprintf(FILE *restrict f, const char *restrict fmt, va_list ap)
 		f->buf_size = sizeof internal_buf;
 		f->wpos = f->wbase = f->wend = 0;
 	}
-	ret = printf_core(f, fmt, &ap2, nl_arg, nl_type);
+	if (!f->wend && __towrite(f)) ret = -1;
+	else ret = printf_core(f, fmt, &ap2, nl_arg, nl_type);
 	if (saved_buf) {
 		f->write(f, 0, 0);
 		if (!f->wpos) ret = -1;