Assertion failure in JavascriptArray.cpp

the following poc cause a assertion failure in "debug" build on ubuntu.

```
const v6 = Array(4294967295);
const v8 = v6["copyWithin"]();
```

Start by reproducing the supplied Array/copyWithin POC in a debug build on Ubuntu, then inspect the assertion path in JavascriptArray.cpp. Done means the POC no longer triggers an assertion and the behavior is covered by an appropriate regression check.

Written by the indexing model from the issue text.