Fix crash in ExternalLHEProducer in case of earler exception

If ExternalLHEProducer throws an exception in begin run, the code will
no longer crash when the module is called in end run.

GeneratorInterface/LHEInterface/plugins/ExternalLHEProducer.cc

@@ -66,7 +66,7 @@ class ExternalLHEProducer : public edm::one::EDProducer<edm::BeginRunProducer,
                                                         edm::EndRunProducer> {
 public:
   explicit ExternalLHEProducer(const edm::ParameterSet& iConfig);
-  virtual ~ExternalLHEProducer();
+  virtual ~ExternalLHEProducer() override;
 
   static void fillDescriptions(edm::ConfigurationDescriptions& descriptions);
 
@@ -81,7 +81,7 @@ class ExternalLHEProducer : public edm::one::EDProducer<edm::BeginRunProducer,
   void executeScript();
   std::unique_ptr<std::string> readOutput();
 
-  virtual void nextEvent();
+  void nextEvent();
 
   // ----------member data ---------------------------
   std::string scriptName_;
@@ -92,7 +92,7 @@ class ExternalLHEProducer : public edm::one::EDProducer<edm::BeginRunProducer,
   unsigned int nThreads_{1};
   std::string outputContents_;
 
-  std::auto_ptr<lhef::LHEReader>		reader_;
+  std::unique_ptr<lhef::LHEReader>	reader_;
   boost::shared_ptr<lhef::LHERunInfo>	runInfoLast;
   boost::shared_ptr<lhef::LHERunInfo>	runInfo;
   boost::shared_ptr<lhef::LHEEvent>	partonLevel;
@@ -267,8 +267,7 @@ ExternalLHEProducer::beginRunProduce(edm::Run& run, edm::EventSetup const& es)
 
   std::vector<std::string> infiles(1, outputFile_);
   unsigned int skip = 0;
-  std::auto_ptr<lhef::LHEReader> thisRead(new lhef::LHEReader(infiles, skip));
-  reader_ = thisRead;
+  reader_ = std::make_unique<lhef::LHEReader>(infiles, skip);
 
   nextEvent();
   if (runInfoLast) {
@@ -496,6 +495,7 @@ void ExternalLHEProducer::nextEvent()
   if (partonLevel)
     return;
 
+  if(not reader_) { return;}
   partonLevel = reader_->next();
   if (!partonLevel)
     return;