Fix interaction between magnitudes and negation #85

1. 
   sipa commented at 9:25 am on November 2, 2014: contributor

   Magnitude m means values are allowed to be up to 2 * 0xFFF…FFF * m, while the argument passed to secp256k1_fe_negate didn’t take the 2 into account. Fix this.
2. Fix interaction between magnitudes and negation

   Magnitude m means values are allowed to be up to 2 * 0xFFF...FFF * m,
   while the argument passed to secp256k1_fe_negate didn't take the 2 into
   account. Fix this.

   7a8e385d74
3. 
   sipa commented at 9:34 am on November 2, 2014: contributor

   @peterdettman I believe you’ll agree with this?
4. 
   peterdettman commented at 3:27 pm on November 2, 2014: contributor

   Yes, agreed!
5. sipa merged this on Nov 3, 2014
6. sipa closed this on Nov 3, 2014

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7. sipa referenced this in commit cc20075ef7 on Nov 3, 2014

Contributors
sipa peterdettman