Fix crash when bogus string input is provided to hl_date_from_string

[sebbernery]

Fix crash when bogus string input is provided to hl_date_from_string or format digest function.

Added a new function in hl.h, hl_to_utf8_len, same as hl_to_utf8 but with a len parameter Updated hl_utf16_to_utf8 to replace '!= end' by '< end' because c can be incremented two times in the loop and it may be an issue with invalid input.

Here is a reproductible example :

segfault_fixed_by_utf8_len.txt

class Main {
    static public function main() {
        final input = Sys.stdin().readLine();
        try {
            var date = Date.fromString(input);
            trace(date);
        } catch (e:haxe.Exception) {
        }
    }
}

$ haxe -main Main -hl out.hl
$ hl out.hl < segfault_fixed_by_utf8_len.txt
SIGNAL 11[Segmentation fault]
$Date.fromString(/usr/share/haxe/std/hl/_std/Date.hx:157)
$ParseDate.main(ParseDate.hx:5)
.init(?:1)
[1]    2231895 segmentation fault

Here is the reproductible example for Shadigest (with the same input):

import haxe.crypto.Sha1;

class Main {
    static function main() {
        final input = Sys.stdin().readLine();
        try {
            trace(Sha1.encode(input));
        } catch (e:haxe.Exception) {
        }
    }
}

$ haxe -main Main -hl out.hl
$ hl out.hl < segfault_fixed_by_utf8_len.txt
SIGNAL 11[Segmentation fault]
haxe.crypto.$Sha1.encode(/usr/share/haxe/std/hl/_std/haxe/crypto/Sha1.hx:28)
$Main.main(Main.hx:8)
.init(?:1)
[1]    2671945 segmentation fault

I found this issue with AFL++ by trying fuzzing on some Haxe std function. Don't hesitate to ask if you need more details (I kept some notes during the debugging process, but it's in french).

I didn't used any LLM in the entire process from writting Haxe script or fixes or this PR.
Have a nice day !

[sebbernery]

I just noted that my fix is incorrect as it can raise a segfault when there was no problem before with the date parsing, I think it comes from this : https://github.com/HaxeFoundation/haxe/blob/development/std/hl/_std/Date.hx#L157 . The length of the string given is multiplied by two, so I think it's not coherent with the real size of the buffer. I don't get why.

I switched the PR in draft mode.

[yuxiaomao]

Note also: on Windows, the try-catch catched the access violation, and it won't access violation if I trace input before Date.fromString