client: Fixed segmentation faults caused by SyncCallback::DeleteBuffers():

- Do not delete memory if ClientRequest was not set yet.
- DeleteBuffers() uses new clear_* methods which set pointer to NULL to allow a double delete.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3024 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534