scorch zap postingsIter reuses nextLocs/nextSegmentLocs
scorch zap postingsIter reuses nextLocs/nextSegmentLocs

The previous code would inefficiently throw away the nextLocs and
would also throw away the []segment.Location slice if there were no
locations, such as if it was a 1-hit postings list.

This change tries to reuse the nextLocs/nextSegmentLocs for all cases.

blevesearch/bleve,blevesearch/bleve,steveyen/bleve,steveyen/bleve