SOLR-5122: Fixed bug in spellcheck.collateMaxCollectDocs.  Eliminates risk of divide by zero, and makes estimated hit counts meaningful in non-optimized indexes. (merge r1514402)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1514408 13f79535-47bb-0310-9956-ffa450edef68