don't de-reference null pointer - fix crash with certain PPTX files
don't de-reference null pointer - fix crash with certain PPTX files

Signed-off-by: Jan Holesovsky <67a28ff7a21deab50d2ee273ee43e6600bf6af60@suse.cz>
Signed-off-by: Cédric Bosdonnat <e1881e47bb73fa679732379945f70e8fb55668a1@free.fr>

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core