Public API of spirv-tools exposes UB to safe code

[dtolnay]

Describe the bug
For example, the following safe program leads to spirv-tools performing Undefined Behavior:

// [dependencies]
// spirv-tools = "0.7.0"

use std::convert::TryFrom;

fn main() {
    let _ = spirv_tools::error::Diagnostic::try_from(0xdeadbeefusize as *mut _);
}

To Reproduce
Put the code above into a src/main.rs, and the dependency on spirv-tools into a Cargo.toml and run using cargo run.

Segmentation fault (core dumped)

Expected behavior
In general it is unsound for a safe API to result in UB. An API that is liable to cause UB should not be callable from safe code.