Fix TypedArray.prototype.at() to handle resized ArrayBuffer correctly

[sr5434]

There is a bug where, when you expand a resizable array buffer backing a typed array, the at method reads from the newly allocated data rather than returning undefined. Below is a minimal reproducer I made:

const buffer = new ArrayBuffer(4 * Int8Array.BYTES_PER_ELEMENT, {maxByteLength: 8 * Int8Array.BYTES_PER_ELEMENT,});
const array = new Int8Array(buffer);
const index = {
    valueOf() {
        buffer.resize(6 * Int8Array.BYTES_PER_ELEMENT);
        return 4;
    },
};
array.at(index); // Node.JS gives undefined, but 0 is returned by QuickJS at master

I checked past PRs/issues, but all I could find was one about shrinking TypedArrays, while this one is about expanding them. I also added a test based on the minimal reproducer above.