if s=0 would create dead-loop

[quatrix]

the main change is adding the check

            // Break if size zero BoxHeader, which can result in dead-loop.
            if s == 0 {
                break;
            }

in stbl.rs, the dead-loop indeed happened for me in production, a user uploaded a currupted mp4 file and the process got stuck in a tight 100% cpu usage loop.

also added this assert in skip_box as a catch all for all other possible bugs, i assume here that correct code won't try skipping box to the same place

    // if we're jumping to the same place, it's probably a bug in the caller
    assert!(size > 0);

all other changes are just applying clippy suggestions to make the tests pass.