Fix SizeStr index out of band

add test for it

Signed-off-by: Jakub Sztandera <kubuxu@protocol.ai>

chain/types/bigint.go

@@ -131,7 +131,7 @@ func (bi BigInt) SizeStr() string {
 	den := big.NewRat(1, 1024)
 
 	var i int
-	for f, _ := r.Float64(); f >= 1024 && 1 < len(sizeUnits); f, _ = r.Float64() {
+	for f, _ := r.Float64(); f >= 1024 && i+1 < len(sizeUnits); f, _ = r.Float64() {
 		i++
 		r = r.Mul(r, den)
 	}

chain/types/bigint_test.go

@@ -2,6 +2,7 @@ package types
 
 import (
 	"bytes"
+	"math/big"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -69,3 +70,11 @@ func TestSizeStr(t *testing.T) {
 		assert.Equal(t, c.out, NewInt(c.in).SizeStr(), "input %+v, produced wrong result", c)
 	}
 }
+
+func TestSizeStrBig(t *testing.T) {
+	ZiB := big.NewInt(50000)
+	ZiB = ZiB.Lsh(ZiB, 70)
+
+	assert.Equal(t, "5e+04 ZiB", BigInt{Int: ZiB}.SizeStr(), "inout %+v, produced wrong result", ZiB)
+
+}