When HelpIndexer::indexDocuments creates lucene::index::IndexWriter
with a long path like
C:\lo\src\build\workdir\longPathTest_123456789012345678901234567890123456789012345678901234567890\instdir\program\..\program\..\user\extensions\bundled\registry\com.sun.star.comp.deployment.help.PackageRegistryBackend\lu149121qyy8a.tmp\da\help.idxl
then CLucene's FSDirectory::FSIndexOutput::FSIndexOutput may fail
and throw, which would unwind, and call FSIndexOutput destructor,
then procede to inherited BufferedIndexOutput destructor, which
calls close(), which calls flush(), which finally calls flushBuffer;
and that one was pure virtual in BufferedIndexOutput, which meant,
that in BufferedIndexOutput destructor, that was a pure virtual
function call, crashing the process.

Patch CLucene to have a default implementation of the function,
usable in its destructor.

Change-Id: I6f84c8cf2bd24b9bb92a71da485089ebf832530a
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/156944
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>