net: NetworkMonitor: gracefully handle backend initialization failure

Change-Id: I1a22fd53e9905b6e5892573d1473d1f4e8969522
Refs: #4668
diff --git a/src/net/detail/network-monitor-impl-noop.hpp b/src/net/detail/network-monitor-impl-noop.hpp
index 919d231..5b1a00d 100644
--- a/src/net/detail/network-monitor-impl-noop.hpp
+++ b/src/net/detail/network-monitor-impl-noop.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
-/**
- * Copyright (c) 2013-2017 Regents of the University of California.
+/*
+ * Copyright (c) 2013-2018 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -33,7 +33,9 @@
 {
 public:
   explicit
-  NetworkMonitorImplNoop(boost::asio::io_service& io) = default;
+  NetworkMonitorImplNoop(boost::asio::io_service&)
+  {
+  }
 
   uint32_t
   getCapabilities() const final
diff --git a/src/net/detail/network-monitor-impl-osx.cpp b/src/net/detail/network-monitor-impl-osx.cpp
index 839faaa..c4e45e6 100644
--- a/src/net/detail/network-monitor-impl-osx.cpp
+++ b/src/net/detail/network-monitor-impl-osx.cpp
@@ -66,13 +66,13 @@
 #include <boost/asio/ip/address.hpp>
 #include <boost/asio/ip/udp.hpp>
 
+NDN_LOG_INIT(ndn.NetworkMonitor);
+
 namespace ndn {
 namespace net {
 
 using util::CFReleaser;
 
-NDN_LOG_INIT(ndn.NetworkMonitor);
-
 class IfAddrs : noncopyable
 {
 public: