Added ensureInitialized() to the Neg() method (#80)

* Added ensureInitialized to the Neg() method,
because it can panic when d.value is nil

* TestDecimal_NegFromEmpty added

decimal.go

@@ -278,6 +278,7 @@ func (d Decimal) Sub(d2 Decimal) Decimal {
 
 // Neg returns -d.
 func (d Decimal) Neg() Decimal {
+	d.ensureInitialized()
 	val := new(big.Int).Neg(d.value)
 	return Decimal{
 		value: val,

decimal_test.go

@@ -936,6 +936,14 @@ func TestDecimal_Neg(t *testing.T) {
 	}
 }
 
+func TestDecimal_NegFromEmpty(t *testing.T) {
+	a := Decimal{}
+	b := a.Neg()
+	if b.String() != "0" {
+		t.Errorf("expected %s, got %s", "0", b)
+	}
+}
+
 func TestDecimal_Mul(t *testing.T) {
 	type Inp struct {
 		a string