[php-src] Issue #21688: SEGV at ext/dom/node.c

Issue: https://github.com/php/php-src/issues/21688
Author: YuanchengJiang

### Description

The following code:

```php
<?php
$dom = Dom\HTMLDocument::createEmpty();
$fusion = $dom;
echo $fusion->C14N() . PHP_EOL;
```

Resulted in this output:
```
/home/phpfuzz/WorkSpace/flowfusion/php-src/ext/dom/node.c:2108:12: runtime error: member access within null pointer of type 'xmlNode' (aka 'struct _xmlNode')
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/phpfuzz/WorkSpace/flowfusion/php-src/ext/dom/node.c:2108:12 in 
```

To reproduce:
```
./php-src/sapi/cli/php  ./test.php
```

Commit:
```
cdc0366bbb366934e91b5461d524226ecb26c891
```

Configurations:
```
CC="clang-12" CXX="clang++-12" CFLAGS="-DZEND_VERIFY_TYPE_INFERENCE" CXXFLAGS="-DZEND_VERIFY_TYPE_INFERENCE" ./configure --enable-debug --enable-address-sanitizer --enable-undefined-sanitizer --enable-re2c-cgoto --enable-fpm --enable-litespeed --enable-phpdbg-debug --enable-zts --enable-bcmath --enable-calendar --enable-dba --enable-dl-test --enable-exif --enable-ftp --enable-gd --enable-gd-jis-conv --enable-mbstring --enable-pcntl --enable-shmop --enable-soap --enable-sockets --enable-sysvmsg --enable-zend-test --with-zlib --with-bz2 --with-curl --with-enchant --with-gettext --with-gmp --with-mhash --with-ldap --with-libedit --with-readline --with-snmp --with-sodium --with-xsl --with-zip --with-mysqli --with-pdo-mysql --with-pdo-pgsql --with-pgsql --with-sqlite3 --with-pdo-sqlite --with-webp --with-jpeg --with-freetype --enable-sigchild --with-readline --with-pcre-jit --with-iconv
```

Operating System:
```
Ubuntu 20.04 Host, Docker 0599jiangyc/flowfusion:latest
```

*This report is automatically generated by [FlowFusion](https://github.com/php/flowfusion)*


### PHP Version

```plain
nightly
```

### Operating System

_No response_