fdo#59428: SwPageDesc::GetUseOn(): mask PD_FIRSTSHARE.
fdo#59428: SwPageDesc::GetUseOn(): mask PD_FIRSTSHARE.

There is at least one call in SwDoc::ChgPageDesc() checks if the result
equals PD_MIRROR.

(regression from fa0f42bafbf24e9141ddee728b160b5ab47077f2)

Change-Id: Id37ca46eec5885f7f5230337d8a8a594845f7842

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