Bug SetString for frElement

[ThomasPiellard]

SetString() doesn't erase the previous value of a frElement if a small number of bits is changed.
This example (r = BN254 scalar field) outputs the same result:

        var y fr.Element
	y.SetString("16406754891999554747479650379038048271643900448173543122927661446988296543616")
	fmt.Println(y.String())
	y.SetString("06406754891999554747479650379038048271643900448173543122927661446988296543616")
	fmt.Println(y.String())

[gbotrel]

func TestString(t *testing.T) {
	var y Element
	y.SetString("16406754891999554747479650379038048271643900448173543122927661446988296543616")
	fmt.Println(y.String())
	if _, err := y.SetString("06406754891999554747479650379038048271643900448173543122927661446988296543616"); err != nil {
		fmt.Println(err)
	}
	fmt.Println(y.String())
}

-->

Element.SetString failed -> can't parse number into a big.Int 06406754891999554747479650379038048271643900448173543122927661446988296543616

[gbotrel]

SetString relies on big.Int underneath, which errors with this input