[iced-x86] Incorrect buffer size calculation in iced-x86

An issue was discovered in the iced-x86 crate through 1.10.3 for Rust. In Decoder::new(), slice.get_unchecked(slice.length()) is used unsafely.

References

• https://nvd.nist.gov/vuln/detail/CVE-2021-38188
• https://github.com/icedland/iced/issues/168
• https://github.com/icedland/iced/commit/3c607a003e03b773108401d109167d1840487dce
• https://rustsec.org/advisories/RUSTSEC-2021-0068.html
• https://github.com/advisories/GHSA-jjx5-3f36-6927