Nightly builds failing since 27th Dec

[kinetiknz]

Looks like the build has been failing on nightly since the 27th Dec weekly cron job. The last good build was the 20th (previous weekly cron job), so that gives us the window for the nightly regression.

Alfredo noticed this in #135.

[kinetiknz]

Most likely a result of this change: rust-lang/rust#46833, which is a great safety improvement for FFI.

So our test was broken, relying on a panic (from assert!) to unwind through mp4parse_free when called from Rust, but triggering that assert via an FFI call would be UB.