...so spell out its single use, locking the appropriate mutex around the access

Change-Id: I8e8f47de1979f5a80cf1ad65e5ec24d25145c463
Reviewed-on: https://gerrit.libreoffice.org/42908Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>