Fix NullEventHandler overloads

Fix some virtual methods of NullEventHandler that were apparently
intended to overload base class methods, but did not. Add override
keyword to prevent these getting out of sync again.

util/parse.cpp

@@ -20,21 +20,21 @@ Params ParseArgs(int argc, char** argv) {
 
 class NullEventHandler : public YAML::EventHandler {
  public:
-  virtual void OnDocumentStart(const YAML::Mark&) {}
-  virtual void OnDocumentEnd() {}
+  void OnDocumentStart(const YAML::Mark&) override {}
+  void OnDocumentEnd() override {}
 
-  virtual void OnNull(const YAML::Mark&, YAML::anchor_t) {}
-  virtual void OnAlias(const YAML::Mark&, YAML::anchor_t) {}
-  virtual void OnScalar(const YAML::Mark&, const std::string&, YAML::anchor_t,
-                        const std::string&) {}
+  void OnNull(const YAML::Mark&, YAML::anchor_t) override {}
+  void OnAlias(const YAML::Mark&, YAML::anchor_t) override {}
+  void OnScalar(const YAML::Mark&, const std::string&, YAML::anchor_t,
+                const std::string&) override {}
 
-  virtual void OnSequenceStart(const YAML::Mark&, const std::string&,
-                               YAML::anchor_t) {}
-  virtual void OnSequenceEnd() {}
+  void OnSequenceStart(const YAML::Mark&, const std::string&, YAML::anchor_t,
+                       YAML::EmitterStyle::value) override {}
+  void OnSequenceEnd() override {}
 
-  virtual void OnMapStart(const YAML::Mark&, const std::string&,
-                          YAML::anchor_t) {}
-  virtual void OnMapEnd() {}
+  void OnMapStart(const YAML::Mark&, const std::string&, YAML::anchor_t,
+                  YAML::EmitterStyle::value) override {}
+  void OnMapEnd() override {}
 };
 
 void parse(std::istream& input) {