Fix error message.

src/libserver/cfg_rcl.c

@@ -1111,8 +1111,15 @@ rspamd_rcl_composite_handler (rspamd_mempool_t *pool,
 
 	if (!rspamd_parse_expression (composite_expression, 0, &composite_expr_subr,
 				NULL, cfg->cfg_pool, err, &expr)) {
-		msg_err ("cannot parse composite expression for %s: %e",
-				composite_name, err);
+		if (err && *err) {
+			msg_err ("cannot parse composite expression for %s: %e",
+				composite_name, *err);
+		}
+		else {
+			msg_err ("cannot parse composite expression for %s: unknown error",
+				composite_name);
+		}
+
 		return FALSE;
 	}