[STORM-3589] Prevent inconsistent results from consecutive hasNext and next calls in LazyNodeSortingIterator

[RuiLi8080]

Prevent the problem that consecutive hasNext() calls will lose elements in nodeIterator without using it.

Prevent the problem that consecutive next() calls will skip to post iterator since nextValueFromNode is null even if nodeIterator still have available elements.

[Ethanlm]

Good catch. But I think there is a bug in this code:

first assume pre and post are both empty to simplify discussion.

if every node is in the skip list, the first call of hasNext will return false (which is correct), the second call of hasNext will return true since nextValueFromNode is no longer null

[RuiLi8080]

@Ethanlm Wow sorry for overlook this. Thanks for pointing out. Fixed and force pushed.

[Ethanlm]

+1 Thanks for the quick fix