Clear the ssh_session a little later to stop receive errors during SSH connect.

Fixed warning: converting to non-pointer type ‘CRYPT_SESSION {aka int}’ from NULL.
Fix the race-condition crash Deuce reported:
<Deuce> #0  0x0000000802478d51 in pthread_mutex_destroy () from /lib/libthr.so.3
<Deuce> #1  0x0000000800d4e4fa in RingBufFree ()
<Deuce> #2  0x0000000800d27148 in passthru_input_thread ()
The output RingBuf can't be destroyed while the passthru_input_thread is still
running.