add small test to fuzz cid creation routines

cid_test.go

@@ -2,6 +2,7 @@ package cid
 
 import (
 	"bytes"
+	"math/rand"
 	"testing"
 
 	mh "github.com/multiformats/go-multihash"
@@ -108,3 +109,12 @@ func TestPrefixRoundtrip(t *testing.T) {
 		t.Fatal("input prefix didnt match output")
 	}
 }
+
+func TestFuzzCid(t *testing.T) {
+	buf := make([]byte, 128)
+	for i := 0; i < 200; i++ {
+		s := rand.Intn(128)
+		rand.Read(buf[:s])
+		_, _ = Cast(buf[:s])
+	}
+}