Only read required field for BOF record

[hudbrog]

There are BIFF8 files that have BOF record with size 8. With current implementation binary.Read fails (silently, cause errors are ignored) because struct is 16 bytes. Although standard does specify BOF to be 16 bytes, I've stumbled upon files like that on numerous occasions.

libxls only reads first 4 bytes as well from BOF.