fix journald startup problem when code is compiled with -DNDEBUG (#4735)

Similar to this patch from here:
http://systemd-devel.freedesktop.narkive.com/AvfCbi6c/patch-0-3-using-assert-se-on-actions-with-side-effects-on-test-cases

If the code is compiled with -DNDEBUG which is the default for
some embedded buildsystems, systemd-journald does not startup
and silently fails.
(cherry picked from commit 9bab3b65)