[PATCH] Add mpi_set_cond.

[NIIBE Yutaka]

On 02/12/2015 01:28 AM, Werner Koch wrote:
> On Wed, 11 Feb 2015 13:47, gniibe at fsij.org said:
> 
>> OK to commit?  I think that it's worth to have those functions in 1.6.
> 
> No concerns from my site.

Thanks.  Reviewing by myself, I found a possible error.  '0UL' will be
problem for LLP64 machine if mpi_limb_t is long long.

I think that it should be like:

  mpi_limb_t mask = ((mpi_limb_t)0) - !!swap;

Fixing these, I committed the patch.
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