Return internal error if no PSK is found when deriving the premaster secret

Signed-off-by: Guilhem Bryant <Guilhem.Bryant@arm.com>

library/ssl_tls.c

@@ -1859,6 +1859,7 @@ int mbedtls_ssl_psk_derive_premaster( mbedtls_ssl_context *ssl, mbedtls_key_exch
          * checked before calling this function
          */
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "should never happen" ) );
+        return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
     }
 
     /*