mul_fft, cleaning up some details of the code

[Torbjörn Granlund]

Marco Bodrato <bodrato at mail.dm.unipi.it> writes:

  This is not really needed to solve a bug.
  The comment before the mpn_mul_fft_decompose function says "We must
  have nl <= 2*K*l", this means that we should never have ((dif = nl -
  Kl) > Kl), and the code in that branch should never be used.

I noticed that at some point too, and have had a patch lying around for
quite a while:

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(Not sure about the separate FIXME change therein; it looks strange to
me now.)

Feel free to clean this up.


-- 
Torbj?rn
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