The `!BKE_previewimg_is_finished()` in `icon_preview_startjob_all_sizes()`
would fail.
Caused by 990bd9acf243ae. We have to be more picky about tagging previews as
unfinished after file read. But we also have to consider files stored in old
versions and set the unfinished tag as needed.
7dc8db7cd1