Fix some memory leaks

src/list.c

@@ -31,6 +31,7 @@ DEFUN(cons, "cons", (LispVal * car, LispVal *cdr), "(car cdr)",
 
 DEFUN(length, "length", (LispVal * list), "(list)", "") {
     // TODO type check
+    // TODO list may be circular
     return MAKE_FIXNUM(list_length(list));
 }