Fix several overflows in box and track processing

[oftheforest]

Add checked arithmetic in several places where the code:

• Reads a length from an open file, and tries to allocate that much memory
• Subtracts from a length, overflows if it's unexpectedly small, and tries to allocate that much memory
• Performs unchecked arithmetic on a value (e.g. to compute a sample_id), and panics (in debug mode)

(da83d3c isn't an overflow per se, but I think it's in the same spirit.)

[alfg]

Hi @oftheforest. Thanks for the PR. Can you provide a description of the changes you are making here? Thank you.

[alfg]

Hi @oftheforest,

Thanks for the PR. Running through some of my local test vectors on the examples and seeing a few issues:

mp4copy example:

cargo run --example mp4copy tears-of-steel-2s-audio.mp4 tears-of-steel-2s-audio-copy.mp4
    Finished dev [unoptimized + debuginfo] target(s) in 0.10s
     Running `target\debug\examples\mp4copy.exe tears-of-steel-2s-audio.mp4 tears-of-steel-2s-audio-copy.mp4`
thread 'main' panicked at 'attempt to subtract with overflow', src\track.rs:322:52
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: process didn't exit successfully: `target\debug\examples\mp4copy.exe tears-of-steel-2s-audio.mp4 tears-of-steel-2s-audio-copy.mp4` (exit code: 101)

mp4sample example:

cargo run --example mp4sample tears-of-steel-2s-audio.mp4
    Finished dev [unoptimized + debuginfo] target(s) in 0.10s
     Running `target\debug\examples\mp4sample.exe tears-of-steel-2s-audio.mp4`
thread 'main' panicked at 'attempt to subtract with overflow', src\track.rs:322:52
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: process didn't exit successfully: `target\debug\examples\mp4sample.exe tears-of-steel-2s-audio.mp4` (exit code: 101)

It seems to only affect an MP4 with a single track. I provided both an audio and video only mp4 below. Latest master branch seems to work OK.

tears-of-steel-2s-audio.mp4

tears-of-steel-video.mp4

[alfg]

Thanks for the fix. I will test again soon.

[alfg]

Thank you, @oftheforest!