Before commit 3840aede releasing
an unlocked SolarMutex was simply ignored, but now it aborts.

So run the LicenseDialog GUI code holding the SolarMutex, as
required.

Change-Id: Id9f5f6146cdfe2b21b68a2cdffc9dd4a527b551c
Reviewed-on: https://gerrit.libreoffice.org/42951Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>