instead of two double-checked locking patterns

Change-Id: Ic232ff44abcba6beda3edf385c2fc6029c04d936
Reviewed-on: https://gerrit.libreoffice.org/52922Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>