tls13: keys: Fix PSK build only case

When deriving the handshake stage master
secret, in the case of a PSK only build,
the only possible key exchange mode is PSK
and there is no ephemeral key exchange
shared secret in that case. Thus do not
error out in that case in the first
phae of the derivation dedicated to the
shared secret.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

library/ssl_tls13_keys.c

@@ -1310,8 +1310,6 @@ int mbedtls_ssl_tls13_key_schedule_stage_handshake( mbedtls_ssl_context *ssl )
             return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
         }
     }
-#else
-    return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
 #endif /* MBEDTLS_KEY_EXCHANGE_SOME_ECDHE_ENABLED */
 
     /*