rust: Allow to remove self inside a hook

[bet4it]

And I do a cargo fmt and cargo clippy on the code.

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After #1480, remove self inside a hook leads to a segmentation fault. This is the PoC:

use std::ffi::c_void;
use unicorn_engine::unicorn_const::{Arch, Mode, Permission, SECOND_SCALE};
use unicorn_engine::Unicorn;

static mut STEP_HOOK: Option<*mut c_void> = None;

fn step_hook(uc: &mut Unicorn<()>, _addr: u64, _size: u32) {
    unsafe {
        if let Some(step_hook) = STEP_HOOK {
            uc.remove_hook(step_hook)
                .expect("Failed to remove step hook");
            STEP_HOOK = None
        }
    }
}

fn main() {
    let x86_code = [
        0x48, 0xB8, 0xEF, 0xBE, 0xAD, 0xDE, 0x00, 0x00, 0x00, 0x00, 0x0F, 0x05,
    ];
    let mut uc = unicorn_engine::Unicorn::new(Arch::X86, Mode::MODE_32)
        .expect("failed to initialize unicorn instance");
    assert_eq!(uc.mem_map(0x1000, 0x4000, Permission::ALL), Ok(()));
    assert_eq!(uc.mem_write(0x1000, &x86_code), Ok(()));
    unsafe {
        STEP_HOOK = Some(
            uc.add_code_hook(1, 0, step_hook)
                .expect("Failed to add code hook"),
        )
    }
    let _ = uc.emu_start(
        0x1000,
        (0x1000 + x86_code.len()) as u64,
        10 * SECOND_SCALE,
        1000,
    );
}

---

Related issue: #1406

[wtdcode]

Merged, thanks!