group: Save a normalize_to_zero in gej_add_ge

[real-or-random]

As discovered by @sipa in #1033.

See commit message for reasoning but note that the infinity handling will be replaced in the second commit again.

[real-or-random]

I think the comment I wrote is wrong, I'll have a look at this.

[real-or-random]

Fixed... (ok that was quick).

[sipa]

ACK e089eecc1e54551287b12539d2211da631a6ec5c