Fix assertion failure in `JavascriptArray.cpp`

[ShortDevelopment]

Fix #6770

[rhuanjl]

Please can you add a test case for this?

[ShortDevelopment]

Sure, but it seems like I’ve created a new assertion failure; I’ll have another look at this…

[ShortDevelopment]

Problem is, that the assertion error only occurred with extremely large arrays. Using the poc of the mentioned issue causes the test to timeout.

[rhuanjl]

The timeouts are on a per JS file basis - does it still timeout if this test is done in a separate file with nothing else?

[rhuanjl]

Could the test complete in 3 minutes? Normally each test file gets a 1 min timeout but tests run with the Slow tag get 3 minutes instead (and are limited to only a couple of the test builds though that's not the point here).

If even 3 minutes is too short I don't know what to do - an Assert we can't test seems pointless. @ppenzin thoughts?

[ppenzin]

I agree, we should try to run it as a separate test and mark as slow (given it does finish in 3 minutes).