named: Add INSIST() after bindkeysfile configuration load to silence scan-build FP

bin/named/server.c

@@ -8098,6 +8098,7 @@ load_configuration(const char *filename, named_server_t *server,
 	INSIST(result == ISC_R_SUCCESS);
 	CHECKM(setstring(server, &server->bindkeysfile,
 	       cfg_obj_asstring(obj)), "strdup");
+	INSIST(server->bindkeysfile != NULL);
 
 	if (access(server->bindkeysfile, R_OK) == 0) {
 		isc_log_write(named_g_lctx, NAMED_LOGCATEGORY_GENERAL,