Editorial: fix incorrect assertion in TA.prototype.{filter,with}

[bakkot]

Fixes #3978. That was closed because it was opened by an LLM without permission but it really is a bug, observable with

let rab = new ArrayBuffer(16, { maxByteLength: 16 });
let ta = new BigInt64Array(rab);

ta.with(0, { valueOf() { rab.resize(8); return 0n; } });

and

let rab = new ArrayBuffer(16, { maxByteLength: 16 });
let ta = new BigInt64Array(rab);

ta.filter(() => { rab.resize(8); return true; });

and

let rab = new ArrayBuffer(16);
let ta = new BigInt64Array(rab);

ta.filter(() => { if (!rab.detached) rab.transfer(); return true; });

Bug report didn't mention detaching but it comes up for filter; afaict it does not for with (because the coercions happen before the IsValidIntegerIndex check, which always returns false for detached buffers).

[github-actions]

The rendered spec preview for this PR is available as a single page at https://tc39.es/ecma262/pr/3979 and as multiple pages at https://tc39.es/ecma262/pr/3979/multipage .

[linusg]

Confirmed that this causes assertion failures in at least Boa and Kiesel, might be good to add test262 coverage.

[bakkot]

might be good to add test262 coverage

Good thought, done.