Fix ssh_handle_key_exchange() timeout

See libssh-mirror#311 for background. But in some case, it's possible to
trigger the code in ssh_handle_key_exchange() to move session state
directly to SSH_SESSION_STATE_AUTHENTICATED. The exit condition for this
function is SSH_SESSION_STATE_AUTHENTICATING though, so when it happens,
ssh_handle_key_exchange() will time out eventually.

The fix is straightforward. Tested with the problematic
client (trilead-ssh2) and made sure the bad condition happened (and not
cause timeout)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
(cherry picked from commit 168302b9d6)

src/server.c

@@ -523,6 +523,7 @@ static int ssh_server_kex_termination(void *s){
   ssh_session session = s;
   if (session->session_state != SSH_SESSION_STATE_ERROR &&
       session->session_state != SSH_SESSION_STATE_AUTHENTICATING &&
+      session->session_state != SSH_SESSION_STATE_AUTHENTICATED &&
       session->session_state != SSH_SESSION_STATE_DISCONNECTED)
     return 0;
   else