[flatbuffers] Unsound casting in flatbuffers

An issue was discovered in the flatbuffers crate before 0.6.1 for Rust. Arbitrary bytes can be reinterpreted as a bool, defeating soundness.

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-25004
• https://github.com/google/flatbuffers/issues/5530
• https://rustsec.org/advisories/RUSTSEC-2019-0028.html
• https://github.com/advisories/GHSA-gx73-2498-r55c