LUCENE-4899: FastVectorHighlihgter failed with StringIndexOutOfBoundsException if a single highlight phrase or term was greater than the fragCharSize producing negative string offsets

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1465041 13f79535-47bb-0310-9956-ffa450edef68