Prevent use after free of TLS context at syslogd(8) shutdown.

Prevent use after free of TLS context at syslogd(8) shutdown.

When splitting the event fields f_ev and f_bufev, disabling some
events was missed.  Callbacks could happen after tls_free().  Call
bufferevent_disable() before f_bufev and struct filed are cleaned.
In some error cases f_bufev might be NULL, add a check before
cleanup.

OK tb@