Re-add comment line from cmsg branch

src/msg.cpp

@@ -78,6 +78,8 @@ int zmq::msg_t::init_data (void *data_, size_t size_, msg_free_fn *ffn_,
     //  If data is NULL and size is not 0, a segfault
     //  would occur once the data is accessed
     assert (data_ != NULL || size_ == 0);
+    
+    //  Initialize constant message if there's no need to deallocate
     if(ffn_ == NULL) {
         u.cmsg.type = type_cmsg;
         u.cmsg.flags = 0;