add dbaupp's noted overflow to the invalid span list

[pnkfelix]

Somehow I left out this overflow PR rust-lang/rust#31281 when I made this list here https://github.com/pnkfelix/collab-docs/blob/master/rust/arith-overflow-buglist.md#invalid-span-constructions

[pnkfelix]

Hat tip to @huonw blog post here for this: http://huonw.github.io/blog/2016/04/myths-and-legends-about-integer-overflow-in-rust/

[pnkfelix]

Oh wait, no, the document did have it, a few lines lower, here: https://github.com/pnkfelix/collab-docs/blob/master/rust/arith-overflow-buglist.md#multiline-error-spans