ctr_add: replace dead with working code.

ctr_add: replace dead with working code.

ctr_add came directly from 2012, and it was always broken.
The original purpose is to add two very large integers.
Still, there is no much to worry about: Threefish 64 bit integers
make it very hard to trigger the old buggy code, and it was only
used to rewind the counter. But it is time to finally fix it.