RUSTSEC-2020-0009: `read_scalar` and `read_scalar_at` allow transmuting values without `unsafe` blocks

[github-actions]

read_scalar and read_scalar_at allow transmuting values without unsafe blocks

Details
Package	flatbuffers
Version	0.6.1
URL	google/flatbuffers#5825
Date	2020-04-11
Unaffected versions	< 0.4.0

The read_scalar and read_scalar_at functions are unsound
because they allow transmuting values without unsafe blocks.

The following example shows how to create a dangling reference:

fn main() {
    #[derive(Copy, Clone, PartialEq, Debug)]
    struct S(&'static str);
    impl flatbuffers::EndianScalar for S {
        fn to_little_endian(self) -> Self { self }
        fn from_little_endian(self) -> Self { self }
    }
    println!("{:?}", flatbuffers::read_scalar::<S>(&[1; std::mem::size_of::<S>()]));
}

See advisory page for additional details.