LUCENE-6049: don't throw exc writing segment if document hit non-aborting exc

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1637528 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-6039: add another null check

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1637542 13f79535-47bb-0310-9956-ffa450edef68