add sanity check: `domain.Cardinality` should match `size := ecc.NextPowerOfTwo(sizeSystem)`

[gbotrel]

          Normally `domain.Cardinality` should match `size := ecc.NextPowerOfTwo(sizeSystem)`, don't know if we should add this sanity check...

Originally posted by @ThomasPiellard in #957 (comment)