Merge pull request #481 from pedro-w/issue-477

Check for NULLs when getting the 'next' message

Author: stevedekorte

libs/iovm/source/IoMessage_opShuffle.c

@@ -383,9 +383,15 @@ void Levels_attach(Levels *self, IoMessage *msg, List *expressions) {
 
  IoMessage *value = IoMessage_deepCopyOf_(DATA(msg)->next);
  IoMessage_rawSetNext_(value, NULL);
-
- IoMessage *rest =
- IoMessage_deepCopyOf_(DATA(DATA(msg)->next)->next);
+ // Step forward twice to the "next" part of the expression,
+ // which may be NULL, and make a copy.
+ IoMessage *rest = DATA(msg->next);
+ if (rest) {
+ rest = DATA(rest->next);
+ }
+ if (rest) {
+ rest = IoMessage_deepCopyOf_(rest);
+ }
  IoMessage_rawSetNext_(slotNameMessage, rest);
  IoMessage_addArg_(slotNameMessage, value);