client: Always check if stat buffer available before accessing it in directory...

client: Always check if stat buffer available before accessing it in directory entry. Added note about it in lib doku and fixed it in FuseAdapter, resolving issue 202.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@2638 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534