listener: use exit_client instead of free_client

listener: use exit_client instead of free_client

As well as leaking a connid and leaving the connection open,
these calls to free_client() leave the client in the unknown_list
causing check_unknowns_list() to crash when either ptr->data
(ptr being the freed client_p->localClient->tnode) is NULL or
when client_p->localClient is NULL.

Flag the client as an IO error so that we don't try to send it
any data (as this is not a normal plaintext connection).