model: Removing assert when interest payload is not zero

In CSMA channels, the minimum required length of a transmitted packet is
46 bytes, resulting in automatically added "payload" to some short
interest packets.  There is no problem of ignoring the assert, as it was
intended only for debug purposes, to ensure that decoding was done
properly.
diff --git a/examples/wscript b/examples/wscript
index 6bc4c80..bbc0f06 100644
--- a/examples/wscript
+++ b/examples/wscript
@@ -7,6 +7,9 @@
     obj = bld.create_ns3_program('ndn-simple', all_modules)
     obj.source = 'ndn-simple.cc'
 
+    obj = bld.create_ns3_program('ndn-csma', all_modules)
+    obj.source = 'ndn-csma.cc'
+
     obj = bld.create_ns3_program('ndn-grid', all_modules)
     obj.source = 'ndn-grid.cc'