tools/xenstored: Avoid dereferencing a NULL pointer if LiveUpdate is failing

tools/xenstored: Avoid dereferencing a NULL pointer if LiveUpdate is failing

In case of failure in do_lu_start(), XenStored will first free lu_start
and then try to dereference it.

This will result to a NULL dereference as the destruction callback will
set lu_start to NULL.

The crash can be avoided by freeing lu_start *after* the reply has been
set.

Fixes: af216a99fb4a ("tools/xenstore: add the basic framework for doing the live update")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>