model: Fixing warning in ndn-face.cc

Thanks to Saran Tarnoi for reporting the problem
diff --git a/model/ndn-face.cc b/model/ndn-face.cc
index 18231f5..57a8c4d 100644
--- a/model/ndn-face.cc
+++ b/model/ndn-face.cc
@@ -184,6 +184,9 @@
           return ReceiveData (Wire::ToData (packet, Wire::WIRE_FORMAT_NDNSIM));
         case HeaderHelper::CONTENT_OBJECT_CCNB:
           return ReceiveData (Wire::ToData (packet, Wire::WIRE_FORMAT_CCNB));
+        default:
+          NS_FATAL_ERROR ("Not supported NDN header");
+          return false;
         }
 
       // exception will be thrown if packet is not recognized
@@ -193,6 +196,8 @@
       NS_FATAL_ERROR ("Unknown NDN header. Should not happen");
       return false;
     }
+
+  return false;
 }
 
 bool