Fix segfault on shutdown during value self-cleanup

It happens when value tries to clenup itself but isolate was already destroyed

Author: pinepain

Diff for: src/php_v8_value.cc

@@ -108,7 +108,7 @@ static void php_v8_value_free(zend_object *object) {
     php_v8_value_t *php_v8_value = php_v8_value_fetch_object(object);
 
     // TODO: check whether we have valid isolate?
-    if (php_v8_value->php_v8_isolate && php_v8_value->persistent && !php_v8_value->persistent->IsEmpty()) {
+    if (PHP_V8_IS_UP_AND_RUNNING() && php_v8_value->php_v8_isolate && php_v8_value->persistent && !php_v8_value->persistent->IsEmpty()) {
         PHP_V8_ENTER_STORED_ISOLATE(php_v8_value);
 
         // TODO: in general, this makes sense only for objects

Diff for: tests/010-no-value-self-cleanup-on-shutdown.phpt

@@ -0,0 +1,25 @@
+--TEST--
+Built-in enum classes
+--SKIPIF--
+<?php if (!extension_loaded("v8")) {
+    print "skip";
+} ?>
+--FILE--
+<?php
+
+/** @var \Phpv8Testsuite $helper */
+$helper = require '.testsuite.php';
+
+$enums = [
+    new V8\StringValue(new V8\Isolate()),
+];
+
+foreach ($enums as $enum) {
+    $rc = new ReflectionClass($enum);
+}
+
+echo 'done', PHP_EOL;
+
+?>
+--EXPECT--
+done