shells/ast-ksh: fix a typo, mark broken on 15

This is broken due to a new compiler on 14 so presumably also on 15.

PR: 275647
Approved by: Marcin Cieślak <saper@saper.info> (maintainer)
Sponsored by: DARPA

(cherry picked from commit c35a419ccf030333c50808944ea171a8daaa8886)