Redis listGetIterator usage code may end up using a NULL pointer

In listGetIterator function comment, common usage code like this:

iter = listGetIterator(list,<direction>);
while ((node = listNext(iter)) != NULL) {
    doSomethingWith(listNodeValue(node));
}

But listGetIterator may return NULL, and listNext doesn't check the parameter "iter", so it's possible that we end up using a NULL pointer.

listNode *listNext(listIter *iter)
{
    listNode *current = iter->next;
    ...
}

Comment From: sundb

Unless OOM, listGetIterator will not return NULL, OOM is already handled in zmalloc.