[BUG] Corrupted UnknownBox data

[LeadAssimilator]

21d46df corrupts UnknownBox data.

This change causes garbage data fields for unknown boxes and should be reverted. BoxHeader reads more than it should (intentionally it seems) and relies on LoadData to seek back to the proper place to correct it. But since that is no longer called, the file position remains too far forward and the read data winds up being offset.

If sanity checking the data size to prevent an exception from ReadBlock is desired, that should be done within LoadData or the exception caught again within LoadData. Also the file null check there might be redundant since LoadData already does it.

The other change in the same commit seems suspect as well, as empty boxes might be useful and important to keep in various cases.