TileRec: Fix warnings seen in LTO build.

Uninitialized variables.

TileCalorimeter/TileRec/src/TileCellToNtuple.cxx

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
 */
 
 //*****************************************************************************
@@ -146,7 +146,7 @@ StatusCode TileCellToNtuple::execute()
 
 
     // step1: read Cells from TDS
-    const CaloCellContainer* CellCnt;
+    const CaloCellContainer* CellCnt = nullptr;
     CHECK( evtStore()->retrieve(CellCnt, m_cellContainer) );
 
     // step2: to fill items in ntuple