[php-src] Issue #20964: UBSAN: fseek() on php://memory with PHP_INT_MIN causes undefined behavior

Issue: https://github.com/php/php-src/issues/20964
Author: arshidkv12

### Description

The following code:

```php
<?php
$stream = fopen('php://memory', 'r+');
$result = fseek($stream, PHP_INT_MIN, SEEK_END);
var_dump($result);```

Resulted in this output:
```
/Users/arshid/Downloads/php-src/main/streams/memory.c:168:45: runtime error: negation of -9223372036854775808 cannot be represented in type 'zend_off_t' (aka 'long long'); cast to an unsigned type to negate this value to itself
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /Users/arshid/Downloads/php-src/main/streams/memory.c:168:45 
[4]    93646 abort      sapi/cli/php z.php
```

But I expected this output instead:
```
-1
```


### PHP Version

```plain
PHP 8.6.0-dev (cli) (built: Jan 18 2026 08:54:52) (NTS DEBUG)
Copyright (c) The PHP Group
Zend Engine v4.6.0-dev, Copyright (c) Zend Technologies
    with Zend OPcache v8.6.0-dev, Copyright (c), by Zend Technologies
```

### Operating System

_No response_