Fix GH-22023 crash during ZTS thread startup (#22027)

Author: shivammathur

Zend/zend.c

@@ -809,6 +809,7 @@ static void executor_globals_ctor(zend_executor_globals *executor_globals) /* {{
 	executor_globals->user_error_handler_error_reporting = 0;
 	ZVAL_UNDEF(&executor_globals->user_error_handler);
 	ZVAL_UNDEF(&executor_globals->user_exception_handler);
+	ZVAL_UNDEF(&executor_globals->last_fatal_error_backtrace);
 	executor_globals->in_autoload = NULL;
 	executor_globals->current_execute_data = NULL;
 	executor_globals->current_module = NULL;