AVIF files with empty iref box fail parsing with older PHP versions

[yguyon]

libavifinfo is used for PHP versions until 8.2 (1) to extract a few image features from AVIF files.

The current version of libavifinfo in WordPress rejects files with an empty 'iref' box. An empty 'iref' box is pointless but is allowed by the specification. See ​https://aomedia.g-issues.chromium.org/issues/466109759#comment3.

(1) ​https://github.com/WordPress/wordpress-develop/blob/5b7a3ad163b5b5aad890c1e7098d746bccf1012d/src/wp-includes/class-avif-info.php#L16

Support AVIF files with an empty 'iref' box.

See the similar fix in PHP-src: ​https://github.com/php/php-src/pull/21250

[SergeyBiryukov]

In 62029:

Media: Correct parsing AVIF files with empty iref box on older PHP versions.

An empty iref box is pointless but is allowed by the specification. This commit imports an upstream fix from libavifinfo.

Follow-up to [57524], [58049].

Props yguyon.
Fixes #64669.