daemon: Protect against double free of callback state while shutting down

daemon: Protect against double free of callback state while shutting down

When CTDB is shut down and monitoring has been stopped, monitor_context
gets freed and all the callback states hanging off it.  This includes
callback state for current_monitor, if the current monitor event has
not yet finished.  As a result, when the shutdown event is called,
current_monitor->callback state is not NULL, but it's actually freed
and it's a dangling reference.

So before executing callback function and freeing callback state check
if ctdb->monitor->monitor_context is not NULL.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
(This used to be ctdb commit 7d8546ee4353851f0543d0ca2c4c67cb0cc75aea)