fix: overflow panics on interesting patches

[sno2]

The patch function would sometimes panic when certain inputs was passed into it. This fixes the issue using the saturating arithmetic functions.

Note: I just realized that this does not affect the crates.io crate because we never released 0.2.0. Therefore, this just removes the vulnerable code in case a release is ever created.

[kornelski]

Maybe overflow should be checked and rejected as a hard error? There's nothing useful to do with overflowing sizes, especially on 64-bit where this can't realistically happen with real data.

LLVM supports allocations only up to isize::MAX, so this could be used as an early check (isize::MAX + isize::MAX doesn't overflow usize).

I agree this should be patched, but I don't think this is a vulnerability. Rust enforces bounds checks on slices either way, so you can only get error or garbage data. With the saturating arithmetic you'll get different garbage data. To cause overflow the attacker has to provide invalid sizes, so the attacker is only mangling their own data.