if we're empty, we're back to initial 'sorted' state

Change-Id: Id85c670ab40d9d0e9848f91f4f2d14212f52c1a1
Reviewed-on: https://gerrit.libreoffice.org/30795
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

svx/source/svdraw/svdmark.cxx

@@ -213,8 +213,8 @@ void SdrMarkList::Clear()
         SdrMark* pMark = GetMark(i);
         delete pMark;
     }
-
     maList.clear();
+    mbSorted = true; //we're empty, so can be considered sorted
     SetNameDirty();
 }