encoding+mgmt: fix definition of INVALID_FACE_ID

Change-Id: I220b183a6f1d007d2c71fcb6491a2c4a487b7673
Refs: #3172
diff --git a/src/mgmt/nfd/face-traits.hpp b/src/mgmt/nfd/face-traits.hpp
index e88e1bc..dee973c 100644
--- a/src/mgmt/nfd/face-traits.hpp
+++ b/src/mgmt/nfd/face-traits.hpp
@@ -47,7 +47,7 @@
   };
 
   FaceTraits()
-    : m_faceId(0)
+    : m_faceId(INVALID_FACE_ID)
     , m_faceScope(FACE_SCOPE_NON_LOCAL)
     , m_facePersistency(FACE_PERSISTENCY_PERSISTENT)
     , m_linkType(LINK_TYPE_POINT_TO_POINT)