TypedArray::from method should protect against null pointers

[jdm]

We should add a testcase to tests/typedarray.rs that uses the typedarray! macro on ptr::null_mut(), and write code that correctly returns an Err value if that's encountered.

Code: src/typedarray.rs

[zakorgy]

@jdm I would like to try this.

[emilio]

Nice! Let us know if you need any help :)

[zakorgy]

Thanks @emilio. If I'm correct, the test case should be look like this:

        rooted!(in(cx) let rval = ptr::null_mut());
        typedarray!(in(cx) let array: Uint8Array = rval.get());
        assert!(array.is_err());

[jdm]

That looks right!