client: Do not access NULL pointers in FuseAdapter::Stop().

This fixes the following error which occurred when a gridmap file is not available:

mount.xtreemfs failed: Failed to open gridmap file: /etc/grid-security/d-grid-xuudb
mount.xtreemfs: /usr/include/boost/smart_ptr/scoped_ptr.hpp:97: T* boost::scoped_ptr<T>::operator->() const [with T = xtreemfs::Client]: Assertion `px != 0' failed.


git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3000 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534