It looks like we never generated correct icon previews for images with
float_rects (non-8bit-images). Images from the report were 16bit pngs.

In this case, icon_preview_startjob would return early (it only
checked if the ImBuf rect was NULL -- which is the case if it has a
rect_float instead). This is not neccessary since icon_copy_rect is
perfectly capable of taking float rects.

Now correct the check and only return early if both rect & rect_float
are NULL.

note: this will not refresh icon previews from existing files
automatically. For this, use File > Data Previews > Clear Data-Block Previews.