LUCENE-4924: DocIdSetIterator.docID() must return -1 when the iterator is not positioned (partially merged from r1468083).

Classes that inherit DocIdSetIterator have been fixed to return -1 when the
iterator is not positioned, but it is still allowed in 4.x to return
NO_MORE_DOCS in such a case.


git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1468101 13f79535-47bb-0310-9956-ffa450edef68