add assert to maybe trick G1GC into not doing this (or at least make the fail explicit)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1593971 13f79535-47bb-0310-9956-ffa450edef68
fix rare test fail

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1593976 13f79535-47bb-0310-9956-ffa450edef68