Update bignum_pure.go - Minor edit -> handling zero division in InvModFr

[RDxR10]

No description provided.

[protolambda]

The idea is that this function signature matches InvModFr in:

• Herumi BLS:

  go-kzg/bls/bignum_hbls.go

  Line 104 in c91cee5

  func InvModFr(dst *Fr, v *Fr) {

• Holiman uint256:

  go-kzg/bls/bignum_hol256.go

  Line 118 in c91cee5

  func InvModFr(dst *Fr, v *Fr) {

• Kilic BLS (default):

  go-kzg/bls/bignum_kilic.go

  Line 113 in c91cee5

  func InvModFr(dst *Fr, v *Fr) {

With build tags the performance of different implementations of field-element operations can be compared in benchmarks etc. since a big part of this repository is FFT related code for data-availability-sampling.

And since native divide by zero behaves the same with a panic, and this error return is a breaking API change, I'm a bit hesitant to merge this. What do you use go-kzg for, and what is the motivation for this PR?

[RDxR10]

I'm just exploring your repo for the sake of learning. This PR is for error handling - might be helpful if used at scale