Change ecp_mul() prototype to allow randomization

(Also improve an error code while at it.)
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 5bedcad..adf5a62 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -2410,7 +2410,8 @@
         if( ( ret = ecdh_calc_secret( &ssl->handshake->ecdh_ctx,
                                       &ssl->handshake->pmslen,
                                        ssl->handshake->premaster,
-                                       POLARSSL_MPI_MAX_SIZE ) ) != 0 )
+                                       POLARSSL_MPI_MAX_SIZE,
+                                       ssl->f_rng, ssl->p_rng ) ) != 0 )
         {
             SSL_DEBUG_RET( 1, "ecdh_calc_secret", ret );
             return( POLARSSL_ERR_SSL_BAD_HS_CLIENT_KEY_EXCHANGE_CS );