The original thinking was that if len(b) < FIXED_SIZE, then that would
be picked up later by the "invalid member length" check. In that case,
this only really *needs* to make sure that the check after this for
"wrong fourcc" wouldn't fail. But if len(b) < FIXED_SIZE, it really is
an invalid size, and should get that error message before even starting
to validate its other qualities.
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