net: NetworkMonitor: gracefully handle backend initialization failure

Change-Id: I1a22fd53e9905b6e5892573d1473d1f4e8969522
Refs: #4668
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: