OString::operator[]: make this consistent with OUString
OString::operator[]: make this consistent with OUString

Change-Id: If63362aba058bbcc0cc6bb1fae1c76005f1291d4

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