Fix error behavior when target has no SOL session
Fix error behavior when target has no SOL session

When a system did not have any custom keep alive, the
code to mark the esssion broken erroneously tries to
dereference.

Change-Id: Ic4efe3c65ee4c09c97ef9f08dc30603c4c15d872

stackforge/pyghmi,openstack/pyghmi,openstack/pyghmi