fix scanf handling of "0" (followed by immediate EOF) with "%x"
fix scanf handling of "0" (followed by immediate EOF) with "%x"

other cases with %x were probably broken too.

I would actually like to go ahead and replace this code in scanf with
calls to the new __intparse framework, but for now this calls for a
quick and unobtrusive fix without the risk of breaking other things.

davidlazar/musl,chardi/musl,chardi/musl,davidlazar/musl,chardi/musl