Fixed some undefined index notice problems

git-svn-id: http://svn.code.sf.net/p/mrbs/code/mrbs/trunk@2905 25f7e783-145e-4f2d-a9f8-dd7182bfe9c7
Made the 'modified_by' field private by default, to match 'create_by'.

git-svn-id: http://svn.code.sf.net/p/mrbs/code/mrbs/trunk@2906 25f7e783-145e-4f2d-a9f8-dd7182bfe9c7