Panic when using read_box

[frewsxcv]

extern crate mp4parse;

use std::io::Cursor;

fn main() {
    let mut c = Cursor::new(b"\x30\x30\x30\x30\x66\x74\x79\x70\x30\x30\x30\x30\x30\x30\x30\x30".to_vec());
    let mut context = mp4parse::MediaContext::new();
    let _ = mp4parse::read_box(&mut c, &mut context);
}

panic discovered using afl.rs

[rillian]

Nice one! This declares a very large number of compatible brands in an ftyp box but doesn't supply them. Since be_fourcc didn't check for short reads, we would hit EOF but return a fourcc value each time. The programme only halts (after creating all 808M?) on the assert at the end of read_box which checks that we've read all the data.

I've added the testcase as 6899348 and a return value check as b154acc which fixes the slow crash.

[rillian]

Sorry, only 202116105 compatible brands. Forgot to divide by 4 for the fourcc width.