fix(num_derive): code duplication error
fix(num_derive): code duplication error

Fixes #240

DonSheddow/num,rust-num/num,rust-num/num-traits,rust-num/num-bigint,rust-num/num-traits,koverstreet/num,rust-num/num-bigint,rust-num/num-integer,vks/num,rust-num/num,DonSheddow/num,rust-num/num-integer,vks/num,rust-lang/num,koverstreet/num,rust-lang/num