Fix unregister_keepalive when no keepalives registered

If no keepalives are registered, return cleanly from an
attempt to unregister rather than raise an exception.

Change-Id: I0064714af4ba8f1b62f9061dc0dc481116c871fe

pyghmi/ipmi/private/session.py

@@ -958,6 +958,8 @@ class Session(object):
         return regid
 
     def unregister_keepalive(self, regid):
+        if self._customkeepalives is None:
+            return
         try:
             del self._customkeepalives[regid]
         except KeyError: