Fix an NPE that comes up in full testing from last commit

git-svn-id: https://zxing.googlecode.com/svn/trunk@2695 59b500cc-1b3d-0410-9834-0bbf25fbcc57
Fix two other AIOOBE from pure barcode mode, update javadoc

git-svn-id: https://zxing.googlecode.com/svn/trunk@2696 59b500cc-1b3d-0410-9834-0bbf25fbcc57