Closes #828. (#829)
Closes #828. (#829)

There was a bug in the LogMergePolicy that was surfacing when there were
segments, but all of the segments were larger than the max limit.

After filtering, the list of segments candidate for merge was 0, and
the code was indexing the first element of an empty Vec.
tantivy-search/tantivy,tantivy-search/tantivy