Blender Git Loki

Fix (unreported) broken ID previews reading.

Recent changes to blendfile reading adding deferred reading of actual
data broke it, we cannot use the nifty `bhead + 1` to access data
anymore, since there is no guaranty that that block hass been fully
read.

Note that there is still one case in `read_file_thumbnail()`, however
loading of blendfile preview itself seems to be working fine... Maybe
@campbellbarton can double check that point (or knows of hands whether
it is OK there)?