Fix crash introduced by incorrect backport 806555e300.

Fix crash introduced by incorrect backport 806555e300.

Commit 7f007e4a04 in master depends on 1476028225, but the latter was
not backported. Therefore 806555e300 (the backport of commit
7f007e4a04) incorrectly used pg_strfold() in a locale where
ctype_is_c.

The fix is to simply have the callers check for ctype_is_c.

Because 7f007e4a04 was only backported to version 18, and because the
commit in master is fine, this fix only exists in version 18.

Reported-by: Александр Кожемякин <a.kozhemyakin@postgrespro.ru>
Discussion: https://postgr.es/m/456f7143-51ea-4342-b4a1-85f0d9b6c79f@postgrespro.ru