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/branches/branch_4x@1593972 13f79535-47bb-0310-9956-ffa450edef68
fix rare test fail

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1593975 13f79535-47bb-0310-9956-ffa450edef68