encoding+mgmt: fix definition of INVALID_FACE_ID

Change-Id: I220b183a6f1d007d2c71fcb6491a2c4a487b7673
Refs: #3172
diff --git a/src/mgmt/nfd/fib-entry.cpp b/src/mgmt/nfd/fib-entry.cpp
index ad10054..e3633aa 100644
--- a/src/mgmt/nfd/fib-entry.cpp
+++ b/src/mgmt/nfd/fib-entry.cpp
@@ -34,7 +34,7 @@
 BOOST_CONCEPT_ASSERT((StatusDatasetItem<FibEntry>));
 
 NextHopRecord::NextHopRecord()
-  : m_faceId(0) // INVALID_FACEID
+  : m_faceId(INVALID_FACE_ID)
   , m_cost(0)
 {
 }