Fix for compilation error

Author: pedro-w

libs/iovm/source/IoMessage_opShuffle.c

@@ -385,9 +385,9 @@ void Levels_attach(Levels *self, IoMessage *msg, List *expressions) {
  IoMessage_rawSetNext_(value, NULL);
  // Step forward twice to the "next" part of the expression,
  // which may be NULL, and make a copy.
- IoMessage *rest = DATA(msg->next);
+ IoMessage *rest = DATA(msg)->next;
  if (rest) {
-  rest = DATA(rest->next);
+ rest = DATA(rest)->next;
  }
  if (rest) {
  rest = IoMessage_deepCopyOf_(rest);