Fix scalbn, scalbln integer overflow

[Joseph S. Myers]

On Fri, 2 Mar 2012, Andreas Jaeger wrote:

> I suggest to add a comment before k=k+n stating why it's not done earlier in
> the code so that the next person might not accidentally "optimize" it.

Committed with the comment:

        /* Now k and n are bounded we know that k = k+n does not
           overflow.  */

-- 
Joseph S. Myers
joseph@codesourcery.com