Fix hang on failure of GSSAPI authentication.

Added a missing pq_push_front (see commit a41eefff4), without which
the SSH_MSG_USERAUTH_FAILURE was being dropped from the userauth
packet queue before returning to the top of the loop which waits for
it to arrive.

ssh.c

@@ -11005,6 +11005,7 @@ static void do_ssh2_userauth(void *vctx)
 			    sfree(s->gss_buf.value);
 			}
 
+                        pq_push_front(&ssh->pq_ssh2_userauth, pktin);
 			break;
 		    }
 		    logevent("GSSAPI authentication initialised");