fdo#49421: do not divide by zero when scrollbar cannot be moved
fdo#49421: do not divide by zero when scrollbar cannot be moved

Change-Id: I63338f6c2cfa3d206260b536a44de3d51fcd5710

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