[3.0.x] ogg : fix demux control to get attachements

This should fix the issue related to vlc-ios#953 (closed) and also ogg artworks parsing for other platforms that use the ogg module.

The issue doesn't appear in vlc 4 code here https://code.videolan.org/videolan/vlc/-/blob/master/modules/demux/ogg.c#L748-750

It seems to be a typo where a ptr is dereferenced twice instead of once.

[Edited]

It seems to be the same fix as 19fc95b3, sounds like a missing back port of the MR !725 (merged)