[std][hl] Fix array pos check, force UInt

[yuxiaomao]

Closes HaxeFoundation/hashlink#723

Seems to be some missing cast from #8408

[Simn]

This entire UInt business here looks pretty wonky, can't we just do a pos < 0 || pos >= length check like normal humans?

[yuxiaomao]

I searched UInt in std and got for example

haxe/std/hl/_std/String.hx

Lines 50 to 55 in fe0d3e3

	public function charCodeAt(index:Int):Null<Int> {
	var idx:UInt = index;
	if (idx >= (length : UInt))
	return null;
	return bytes.getUI16(index << 1);
	}

So I thought it's the "normal" usage. Can change to pos < 0 || pos >= length too if you prefer.

UInt seems to save a comparison (using OJULt)

[Simn]

Doesn't this just work incidentally because something like -1 becomes a large number that then exceeds length? I don't see how this is sound because with a sufficiently small number you could end up with something that's < length after casting.

[yuxiaomao]

Negative Int has the most significant bit at 1, with INT_MIN it's 0x80000000 and is always bigger (unsigned operation) than a normal length ranged from 0 to INT_MAX (0x7FFFFFFF)