When data-block/asset previews were not stored in the thumbnail cache
yet (or were outdated), we'd read them from .blend files. This could
lead to random crashes (but quite reliable with a small number of
previews to be read).
Wasn't clearing runtime memory which could lead to the
`PRV_TAG_DEFFERED` bit being set. This meant we would try to free
deferred preview data since eefee47a8a, which was just garbage memory.
f20853a6c9