mail/courier-imap: Apply hack to fix reported runtime issue

A runtime issue has been reported at [1] which causes courier-imap
to not work properly.

Experimenting shows this issue disappears by downgrading
devel/libunistring to previous version 1.2, but no clear reason why
has been uncovered.

This commit applies a patch similar to what is suggested in [1]
that makes it work as expected.

[1] https://lists.freebsd.org/archives/freebsd-questions/2025-February/006307.html

Reported by: Benjamin Lutz <mail@maxlor.com>