fix for "signed overflow is undefined" warning
fix for "signed overflow is undefined" warning

introduced by my String->OUString conversions

Change-Id: I237c8fcf655d6f39f04512d1a386d0b6631f53c3

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