This was likely never an issue so far, as no ID type has ever been
removed (that was written in blendfiles at least), and no new BHead type
has been added since ages.
However, current behavior of the main loop in the readfile code (in
`blo_read_file_internal`) is to consider any BHead type which is not
explicitely known and handled, to be an ID BHead, and directly try to
load it as such.
This commit addresses that potential issue by validating that a BHead
code actually matches a known ID type before trying to use it to read an
ID from the file.
Issue identified while working on designs for future 5.0 forward
compatibility breaking changes in blendfile format itself.
Pull Request: https://projects.blender.org/blender/blender/pulls/129693
6ba58a8303