runtime: typed-array out-of-bounds index returns 0 instead of undefined

[proggeramlug]

Symptom

Reading a typed array out of bounds returns 0 instead of undefined:

const t = new Uint8Array([5, 6, 7]);
console.log(t[9]);          // node: undefined   perry: 0
const a: any = t;
console.log(a[9]);          // node: undefined   perry: 0

Reproduces for both a statically-typed Uint8Array receiver and an any-typed
one (so it is not generator/CPS-specific). In-bounds reads are correct
(t[0] === 5).

Notes

Spec: an integer-indexed exotic object returns undefined for a
CanonicalNumericIndexString that is out of [0, length). The dynamic path
(js_dyn_index_get → js_typed_array_index_get_dynamic) and the inline
fast/slow typed-array get appear to fall through to a 0 default on OOB rather
than undefined.

Found tangentially while fixing #6067 (generator local string OOB index);
independent of that (string-only) fix. Low urgency — filing for tracking.