Fix CVE-2021-26937 for real: segfaults by displaying some UTF-8 characters

This is a recommit of r565281 fixing a typo in r565281, causing
a regression.

CVE-2021-26937 segfaults when displayingsome UTF-8 characters
described in
https://lists.gnu.org/archive/html/screen-devel/2021-02/msg00000.html.

PR: 253515
Reported by: daniel.engberg.lists at pyret.net
Obtained from: https://build.opensuse.org/request/show/871482
MFH: 2020Q1
Security: CVE-2021-26937