Unsafe Rust: Integer overflow is not UB

[hurryabit]

Slide https://google.github.io/comprehensive-rust/unsafe-deep-dive/introduction/may_overflow.html in the Unsage Rust course claims that integer overflow is UB in release mode. This is simply not true. See https://doc.rust-lang.org/book/ch03-02-data-types.html#integer-overflow for details.

[mgeisler]

Yeah, this needs fixing.