Partition.java does not check the validity of input iterators

[emopers]

Partition.java calls j.previous() on java.util.Iterator j and i.next() on java.util.Iterator i
without checking if there are any elements to iterate over. Because the method is public and the iterators are obtained from inputs, they could be invalid (e.g., an empty list) and lead to an exception. This pull request adds a hasNext() and a hasPrevious() check.

This violation is present in other parts of this class as well. If this fix is acceptable to you, we can
submit another pull request with fixes for all other cases.

[ctrueden]

Looks good to me! What do you think, @tpietzsch?

[ctrueden]

I retract my LGTM; see #259 (comment).

[ctrueden]

In #259 (comment), @tpietzsch wrote:

it should rather check i.nextIndex() < j.previousIndex() or something (I would need to think about the exact condition). Unfortunately, there is no way to check whether i and j are iterators into the same list, which is also an expectation about the input.