I can't reproduce this on release or debug builds. But from the strack
traces provided in a couple reports, it looks like the runtime pointer
is null. This makes sense because BKE_previewimg_blend_read is not
called here which is what allocates the runtime struct on file read.
To maintain a "valid" PreviewImage in more code and avoid the
complexity of making the runtime pointer optional, just allocate
it in BLO_blendhandle_get_preview_for_id.
ff4948fc08