If you close a session window with an associated SSH back end, the
back end may call back to notify_remote_exit() from ssh_free(), which
queues a new top-level callback citing the inst structure we were
about to delete.
We could fix this by introducing a special 'moribund' flag which
inhibits notify_remote_exit from queueing a callback, but far easier
is to move the delete_callbacks_for_context() call to _after_ all
subsidiary things have been cleaned up, so that any last-minute
callbacks they might schedule will be promptly unscheduled again
before they do any damage.
f212e2cbea