...which had accidentally been broken with
87707670 "Make OUStringLiteral more useful".
(std::strlen unfortunately isn't constexpr, so need to use an explicit loop
instead.)

Change-Id: I9a820e2940b99a0c37124477810ef879d82c8325
Reviewed-on: https://gerrit.libreoffice.org/59344
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>