demux: image: fix JFIF detection

A JFIF does not necessarily has an ICC Profile, and especially not usually directly following the SOI, as the spec requires SOI followed by the APP0 JFIF marker. So while this new detection logic fixes out-of-spec files that have a (small) ICC profile before the APP0 JFIF, it breaks most other properly structured ones.

This makes the APP0 ICC profile optional and continues looking for the APP0 JFIF regardless.

Regression from 5ffd36ff