[UB] `CtlType::from(16)` builds an invalid enum outside FreeBSD

[fereidani]

Hey, I've found this while scanning the top downloaded crates with my UB static analyzer.

src/ctl_type.rs:41:

impl std::convert::From<u32> for CtlType {
    fn from(t: u32) -> Self {
        assert!(t <= 16);
        unsafe { std::mem::transmute(t) }
    }
}

The guard allows 16, but discriminant 16 is Temperature, which only exists under #[cfg(target_os = "freebsd")]. On Linux or macOS the valid discriminants stop at 15, so CtlType::from(16) transmutes into a value the enum does not have. It is a safe public conversion, so safe code produces an invalid value.

#[test]
fn ctl_type_from_16_is_invalid_off_freebsd() {
    let t = sysctl::CtlType::from(16u32);
    let _ = format!("{:?}", t);
}

$ cargo +nightly miri test

test ctl_type_from_16_is_invalid_off_freebsd ... error: Undefined Behavior: constructing invalid value of type sysctl::CtlType: at .<enum-tag>, encountered 0x00000010, but expected a valid enum tag
  --> sysctl-0.7.1/src/ctl_type.rs:44:18
   |
44 |         unsafe { std::mem::transmute(t) }
   |                  ^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here

Fix would be to put the bound behind the same cfg as the variant, so it is t <= 16 on FreeBSD and t <= 15 elsewhere. Better still, match on the value and return CtlType::None or an error for anything unknown, which drops the transmute entirely.