MuonRPC_CnvTools: Fix uninitialized class member.

Fixes crash seen in reco.


Former-commit-id: 024a6963

MuonSpectrometer/MuonCnv/MuonRPC_CnvTools/src/RpcRdoToPrepDataTool.cxx

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
 */
 
 
@@ -85,7 +85,8 @@ Muon::RpcRdoToPrepDataTool::RpcRdoToPrepDataTool( const std::string& type, const
   declareProperty("OutputCollection",          	m_rdoContainerKey = std::string("RPCPAD"),"RpcPadContainer to retrieve");
   declareProperty("TriggerOutputCollection", 	m_rpcPrepDataContainerKey = std::string("RPC_Measurements"),"Muon::RpcPrepDataContainer to record");
   declareProperty("InputCollection", 		m_rpcCoinDataContainerKey = std::string("RPC_triggerHits"),"Muon::RpcCoinDataContainer to record");
-  
+
+  m_rpcCoinDataContainer = nullptr;
 }
 
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