fix the panic in TextInput Secure

[xMAC94x]

use iced::widget::text_input::Value;

pub fn main() {
    let clear_value = Value::new("abcd");
    let secure_value = clear_value.secure().to_string();

    // Voxygen's custom paragraph hit-testing returns a character index.
    // For a click near the first masked bullet, this can be 1.
    let voxygen_hit_test_offset = 1;

    println!("masked value: {secure_value:?}");
    println!("masked bytes: {}", secure_value.len());
    println!("voxygen hit-test character offset: {voxygen_hit_test_offset}");

    // This is the vulnerable operation from iced_widget 0.14:
    //
    //   &value[..char_offset.min(value.len())]
    //
    // `voxygen_hit_test_offset` is a character offset, but `str` slicing expects
    // a byte offset. Since `•` is three bytes in UTF-8, offset 1 lands inside it.
    let _prefix = &secure_value[..voxygen_hit_test_offset.min(secure_value.len())];

    unreachable!("the slice above should panic before this point");
}

fixes #3288