chunking: Handle low amount of high-size packages

[travier]

If we have a low amount of high-size packages then we will drain with an index higher than the size of the Vec which triggers a panic.

Fixes: coreos/rpm-ostree#4646

[travier]

Note: Untested yet.

[travier]

thread 'chunking::test::test_advanced_packing' panicked at 'assertion failed: high_size.len() < limit_hs_bins', lib/src/chunking.rs:470:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Found file: var/foo
Found file: var/foo
Found file: var/lib/nested/foo
thread 'chunking::test::test_packing_fcos' panicked at 'assertion failed: high_size.len() < limit_hs_bins', lib/src/chunking.rs:470:5

Looks like this assert just found another off by one in my change.

[cgwalters]

I think this makes sense...but man, we're going to need a lot more tests here.

[travier]

Verified in travier/ostree-containers#1 that this fixes this issue.