RUSTSEC-2020-0012 Relies on undefined behavior of `char::from_u32_unchecked`

The Windows implementation of this crate relied on the behavior of std::char::fromu32unchecked when its safety clause is violated. Even though this worked with Rust versions up to 1.42 at least, that behavior could change with any new Rust version, possibly leading a security issue. The flaw was...