crypto/bn256: improve bn256 fuzzer (#21815)

* crypto/cloudflare: fix nil deref in random G1/G2 reading

* crypto/bn256: improve fuzzer

* crypto/bn256: fix some flaws in fuzzer

crypto/bn256/cloudflare/bn256.go

@@ -23,7 +23,7 @@ import (
 func randomK(r io.Reader) (k *big.Int, err error) {
 	for {
 		k, err = rand.Int(r, Order)
-		if k.Sign() > 0 || err != nil {
+		if err != nil || k.Sign() > 0 {
 			return
 		}
 	}