Regression from commit 707f787c
It didn't ensure that we store mergelib when required, thus still
trying to get function from stubs and failing (with mergelibs
enabled), leading to SEGFAULT later.

Change-Id: Ib15528795f3d65b1ae5c31be2aa5284ce5ff04cf
Reviewed-on: https://gerrit.libreoffice.org/52428Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/52453