Fix the browser thumbnails when file is not indexed by the ML

It was due to the fact that we try to find a ML media corresponding to the file. As it fails when the media is not indexed, a new MediaWrapper was created with id 0. In that case, the BitmapCache key was the same for all the files, leading to a cache collision.

Fixes #2358 (closed)

changed milestone to %3.5.x

added UI bug labels

requested review from @Skantes

approved this merge request

added 2 commits

• ce3e7978 - 1 commit from branch videolan:master
• 5dce5a96 - Fix the browser thumbnails when file is not indexed by the ML

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enabled an automatic merge when the pipeline for 5dce5a96 succeeds

merged