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 fa0f42ba)

Change-Id: Id37ca46eec5885f7f5230337d8a8a594845f7842

sw/inc/pagedesc.hxx

@@ -292,6 +292,7 @@ inline UseOnPage SwPageDesc::GetUseOn() const
     UseOnPage eRet = eUse;
     eRet = (UseOnPage) (eRet & nsUseOnPage::PD_NOHEADERSHARE);
     eRet = (UseOnPage) (eRet & nsUseOnPage::PD_NOFOOTERSHARE);
+    eRet = (UseOnPage) (eRet & nsUseOnPage::PD_NOFIRSTSHARE);
     return eRet;
 }