Likely GMP bug

[Torbjörn Granlund]

As I am traveling I haven't had a chance to take a closer look, but
perhaps this bug is the result of the slightly unconventional code style
user here?  Jumping into loops and pseudo-dead if statement bodies might
not be the best style for avoiding subtle bugs.

I think we should rewrite relevant parts and avoid these constructs.

-- 
Torbjörn
Please encrypt, key id 0xC8601622