JiveXML: fix cppcheck uninitialized variable warnings

graphics/GeometryJiveXML/GeometryJiveXML/MuonGeometryWriter.h

 /*
-  Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef JIVEXML_MUONGEOMETRYWRITER
@@ -167,7 +167,7 @@ namespace JiveXML
     bool equalLength(double a, double b) const ;
 
      /** Pointer to the muon detector manager (GeoModel) */
-    const MuonGM::MuonDetectorManager* m_muon_manager ;
+    const MuonGM::MuonDetectorManager* m_muon_manager{nullptr} ;
 
     /** Maximum deviation from the reference value before the station is considered different. */
     static const double m_smallAngle, m_smallDistance ;

graphics/JiveXML/JiveXML/ONCRPCServerSvc.h

 /*
-  Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef JIVEXML_ONCRPCSERVERSVC_H
@@ -97,10 +97,10 @@ namespace JiveXML {
     EventStreamMap m_eventStreamMap;
 
     //A mutex (mutual exclusive) lock for the data map
-    mutable pthread_mutex_t m_accessLock ATLAS_THREAD_SAFE;
+    mutable pthread_mutex_t m_accessLock ATLAS_THREAD_SAFE{};
 
     //A handle to the server thread
-    pthread_t m_ServerThreadHandle;
+    pthread_t m_ServerThreadHandle{};
 
     //Once this flag is set to false, the thread will stop after handling its
     //last request