An XBAT may not be fully used, and remaining entries may be 0 not POIFSConstants.UNUSED_BLOCK, so avoid trying to process any that we know aren't used. Fixes bug #53810

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1442095 13f79535-47bb-0310-9956-ffa450edef68