Avoids assertion with threaded cell computations for tdf#100811/2.

Change-Id: I2dc0fd80595ccec1824c8675fc97cfc763b88659
Reviewed-on: https://gerrit.libreoffice.org/54080Reviewed-by: Dennis Francis <dennis.francis@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>