Latest commit introduces undefined behavior in hgcd2.c

[Hans Åberg]

> On 18 Sep 2019, at 21:37, Niels Möller <nisse at lysator.liu.se> wrote:
> 
> …a problem, at least in theory, if it is "undefined behavior". I'm afraid I
> don't know these fine details of the C spec by heart.

It is undefined behavior according to the section “Shift operators” in
  https://en.cppreference.com/w/c/language/operator_arithmetic