Use NDN_CXX_UNREACHABLE where appropriate

Change-Id: Idb3d8eade91c9e4d6e09f45a54af170f9c74ff12
diff --git a/tools/nfdc/command-definition.cpp b/tools/nfdc/command-definition.cpp
index 4d92ca7..d421d11 100644
--- a/tools/nfdc/command-definition.cpp
+++ b/tools/nfdc/command-definition.cpp
@@ -90,8 +90,7 @@
     case ArgValueType::ROUTE_ORIGIN:
       return "origin";
   }
-  BOOST_ASSERT(false);
-  return "";
+  NDN_CXX_UNREACHABLE;
 }
 
 CommandDefinition::CommandDefinition(const std::string& noun, const std::string& verb)
@@ -247,8 +246,7 @@
   switch (valueType) {
     case ArgValueType::NONE:
     case ArgValueType::ANY:
-      BOOST_ASSERT(false);
-      return {};
+      break;
 
     case ArgValueType::BOOLEAN:
       return parseBoolean(token);
@@ -289,8 +287,7 @@
       return boost::lexical_cast<RouteOrigin>(token);
   }
 
-  BOOST_ASSERT(false);
-  return {};
+  NDN_CXX_UNREACHABLE;
 }
 
 } // namespace nfdc