tcptls: when disabling a server port, we should set the accept_fd to -1.

tcptls: when disabling a server port, we should set the accept_fd to -1.

If we don't set this to -1 if the structure can be potentially re-used
later then it's possible that we'll issue a close() on an unrelated file
descriptor, breaking asterisk in other interesting ways.

I believe this to be an unlikely scenario, but it costs nothing to be
safe.

Signed-off-by: Jaco Kroon <jaco@uls.co.za>