DocSetBase.getTopFilter() could throw out-of-bounds exception

If you have some segments at the end of your index that are not matched in the
docset, then a reader over these segments could have a docBase that is greater
than the size of the docsets Bits.  Check for this and return an empty iterator.
Merge remote-tracking branch 'flax-lucene-solr-intervals/positions' into bb

flax-lucene-intervals commit id: b6e195c139abbb26e729f6e572c481138a8e917b