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

commit ae922863c4 followup

Change-Id: Ia9b8749880cc8c2b83dd59b4d7325cd666881c46
Reviewed-on: https://gerrit.libreoffice.org/30837
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

svx/source/svdraw/svdmark.cxx

@@ -336,6 +336,8 @@ void SdrMarkList::DeleteMark(size_t nNum)
     {
         maList.erase(maList.begin() + nNum);
         delete pMark;
+        if (maList.empty())
+            mbSorted = true; //we're empty, so can be considered sorted
         SetNameDirty();
     }
 }