properties.pSGlobalShell can be legitimately 0 here.

No testcase, for some reason the export does not crash if it's done by
CppunitTest_sw_filters_test or CppunitTest_sw_odfexport; but it does
crash with '--headless --convert-to odt kde140265-1.doc'.

Change-Id: I50936f9e5236763cc987aba14d8fc897c4b789bf
(cherry picked from commit 4cd3f9a7)