Fix a bug in CKE construction for PSK

In plain PSK we don't need to do anymore construction after the preamble.
We weren't detecting this case and treating it as an unknown cipher.

Reviewed-by: Rich Salz <rsalz@openssl.org>

ssl/statem/statem_clnt.c

@@ -2496,7 +2496,7 @@ int tls_construct_client_key_exchange(SSL *s)
     } else if (alg_k & SSL_kSRP) {
         if (!tls_construct_cke_srp(s, &pkt, &al))
             goto err;
-    } else {
+    } else if (!(alg_k & SSL_kPSK)) {
         ssl3_send_alert(s, SSL3_AL_FATAL, SSL_AD_HANDSHAKE_FAILURE);
         SSLerr(SSL_F_TLS_CONSTRUCT_CLIENT_KEY_EXCHANGE, ERR_R_INTERNAL_ERROR);
         goto err;