Fix SelectArrayIterator.MoveNext after Dispose

[jamesqo]

Just realized that there was a small issue with my changes in #11841: if someone tries to MoveNext on a SelectArrayIterator after it's Disposed, then an IOOR exception will be generated because we try to access index -2. This is the fix.

Repro:

var s = new int[1].Select(a => a);
using (var e = s.GetEnumerator())
{
    e.MoveNext();
    e.MoveNext(); // Disposes
    e.MoveNext(); // before called SelectArrayIterator.Dispose again, now throws
}

cc @stephentoub, @VSadov

[stephentoub]

Thanks. Can you please add a test?

[jamesqo]

@stephentoub All done. I also made the fix a bit shorter.

[stephentoub]

Assert.False(e.MoveNext());

[stephentoub]

Why a TheoryData rather than a normal MemberData?

[jamesqo]

I like TheoryData a bit better because it guarantees type/arity safety, e.g. you can't pass in the wrong type or the wrong number of arguments to it.

[stephentoub]

Understood. But we have close to zero use of it across the repo, whereas InlineData and MemberData are used pervasively. I would rather we not diverge from that except where we have very good reasons for doing so. Different people can prefer different things.

[jamesqo]

@stephentoub Ok.

[stephentoub]

Thanks

[stephentoub]

Thanks, @jamesqo.