Check the return value of read() in be_fourcc().
Check the return value of read() in be_fourcc().

The std::io::Read::read method doesn't return an error
if it was only able to partially fill the given buffer.
Instead it returns an Ok(bytes_read) value, much like
libc::fread(). We weren't checking this, which meant
that we could return an invalid Ok([u8; 4]) instead of EOF
if we were called at the end of a buffer.

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