remove memory leak in function parser (bug. no 478),

second case

src/frontend/parse.c

@@ -307,6 +307,7 @@ struct pnode *PP_mkfnode(const char *func, struct pnode *arg)
         /* Give the user-defined functions a try. */
         q = ft_substdef(func, arg);
         if (q) { /* found */
+            free_pnode(arg);
             return q;
         }
     }