HGTD_GeoModel: Fix warnings seen when LTO is enabled.

    
Uninitialized pointer passed to SG::retrieve.

HighGranularityTimingDetector/HGTD_DetDescr/HGTD_GeoModel/src/HGTD_DetectorTool.cxx

 /*
-  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
 */
 
 // includes
@@ -43,7 +43,7 @@ StatusCode HGTD_DetectorTool::create() {
     ATH_CHECK(detStore()->retrieve(idHelper, "HGTD_ID"));
     m_athenaComps.setIdHelper(idHelper);
 
-    GeoModelExperiment *theExpt;
+    GeoModelExperiment *theExpt = nullptr;
     ATH_CHECK(detStore()->retrieve(theExpt, "ATLAS"));
 
     // The * converts a ConstPVLink to a ref to a GeoVPhysVol, the & takes the address of the GeoVPhysVol