[php-src] Issue #21242: `getimagesizefromstring()` with AVIF image returns false sometimes

Issue: https://github.com/php/php-src/issues/21242
Author: carestad

### Description

The following code:

```php
<?php
var_dump(getimagesizefromstring(Http::withHeader('accept', 'image/avif')->get('https://www.ikea.com/no/no/images/products/alvine-spets-lette-gardiner-1-par-offwhite-med-stanglomme__0099780_pe242082_s5.jpg')->body()));
```

_(Using the Laravel Http facade here to simplify setting the `accept` header to `image/avif` since IKEA serves images based on what browsers prefer)_

Resulted in this output:
```
false
```

But I expected this output instead:
```
[
    0 => 1400,
    1 => 1400,
    2 => 2,
    3 => "width="1400" height="1400"",
    "bits" => 8,
    "channels" => 3,
    "mime" => "image/avif",
    "width_unit" => "px",
    "height_unit" => "px",
  ]
```

This does not seem to work in 8.4.18 nor 8.5.3. I have not tried older versions in 8.3 or 8.2 though.

This has been tested on a docker container where GD is compiled like this:

```
FROM php:8.5.3-alpine3.23

RUN apk add --no-cache \
    $PHPIZE_DEPS \
    bash \
    git \
    freetype-dev \
    libjpeg-turbo-dev \
    libwebp-dev \
    libpng-dev \
    libavif-dev \
    zip \
    unzip \
    libzip-dev \
    bind-tools \
    libpq-dev \
    linux-headers \
  && docker-php-ext-configure gd --with-freetype --with-jpeg --with-webp --with-avif \
  && docker-php-ext-install -j$(nproc) \
    gd exif bcmath pcntl pdo_pgsql zip sockets
```

Other AVIF images like [this one](https://raw.githubusercontent.com/test-images/avif/refs/heads/main/202105/squoosh/losssless7/cs-gray-7f7f7f.avif) works well, so I assume this only applies to certain AVIF encoded images.

### PHP Version

```plain
8.5.3

More info from `phpinfo()`:


PHP Version => 8.5.3

System => Linux 46fecb3a11d1 6.18.7-061807-generic #202601231045 SMP PREEMPT_DYNAMIC Fri Jan 23 11:25:00 UTC 2026 x86_64
Build Date => Feb 13 2026 18:34:10
Build System => Linux - Docker
Build Provider => https://github.com/docker-library/php
Configure Command =>  './configure'  '--build=x86_64-linux-musl' '--sysconfdir=/usr/local/etc' '--with-config-file-path=/usr/local/etc/php' '--with-config-file-scan-dir=/usr/local/etc/php/conf.d' '--enable-option-checking=fatal' '--with-mhash' '--with-pic' '--enable-mbstring' '--enable-mysqlnd' '--with-password-argon2' '--with-sodium=shared' '--with-pdo-sqlite=/usr' '--with-sqlite3=/usr' '--with-curl' '--with-iconv=/usr' '--with-openssl' '--with-readline' '--with-zlib' '--enable-phpdbg' '--enable-phpdbg-readline' '--with-pear' 'build_alias=x86_64-linux-musl' 'PHP_UNAME=Linux - Docker' 'PHP_BUILD_PROVIDER=https://github.com/docker-library/php'
Server API => Command Line Interface
Virtual Directory Support => disabled
Configuration File (php.ini) Path => /usr/local/etc/php
Loaded Configuration File => (none)
Scan this dir for additional .ini files => /usr/local/etc/php/conf.d
Additional .ini files parsed => /usr/local/etc/php/conf.d/docker-php-ext-bcmath.ini,
/usr/local/etc/php/conf.d/docker-php-ext-exif.ini,
/usr/local/etc/php/conf.d/docker-php-ext-gd.ini,
/usr/local/etc/php/conf.d/docker-php-ext-pcntl.ini,
/usr/local/etc/php/conf.d/docker-php-ext-pdo_pgsql.ini,
/usr/local/etc/php/conf.d/docker-php-ext-sockets.ini,
/usr/local/etc/php/conf.d/docker-php-ext-sodium.ini,
/usr/local/etc/php/conf.d/docker-php-ext-zip.ini,
/usr/local/etc/php/conf.d/docker-php.ini

PHP API => 20250925
PHP Extension => 20250925
Zend Extension => 420250925
Zend Extension Build => API420250925,NTS
PHP Extension Build => API20250925,NTS
PHP Integer Size => 64 bits
Debug Build => no
Thread Safety => disabled
Zend Signal Handling => enabled
Zend Memory Manager => enabled
Zend Multibyte Support => provided by mbstring
Zend Max Execution Timers => disabled
IPv6 Support => enabled
DTrace Support => disabled

Registered PHP Streams => https, ftps, compress.zlib, php, file, glob, data, http, ftp, phar, zip
Registered Stream Socket Transports => tcp, udp, unix, udg, ssl, tls, tlsv1.0, tlsv1.1, tlsv1.2, tlsv1.3
Registered Stream Filters => zlib.*, convert.iconv.*, string.rot13, string.toupper, string.tolower, convert.*, consumed, dechunk

(...)

gd

GD Support => enabled
GD Version => bundled (2.1.0 compatible)
FreeType Support => enabled
FreeType Linkage => with freetype
FreeType Version => 2.14.1
GIF Read Support => enabled
GIF Create Support => enabled
JPEG Support => enabled
libJPEG Version => 8
PNG Support => enabled
libPNG Version => 1.6.54
WBMP Support => enabled
XBM Support => enabled
WebP Support => enabled
BMP Support => enabled
AVIF Support => enabled
TGA Read Support => enabled
```

### Operating System

Alpine 3.23.3