Fix binary marshalling of decimal zero value (#253)

* Fix binary marshalling of zero value
* Adapt the error message of BinaryUnmarshal

decimal.go

@@ -1353,10 +1353,10 @@ func (d Decimal) MarshalJSON() ([]byte, error) {
 // UnmarshalBinary implements the encoding.BinaryUnmarshaler interface. As a string representation
 // is already used when encoding to text, this method stores that string as []byte
 func (d *Decimal) UnmarshalBinary(data []byte) error {
-	// Verify we have at least 5 bytes, 4 for the exponent and at least 1 more
-	// for the GOB encoded value.
-	if len(data) < 5 {
-		return fmt.Errorf("error decoding binary %v: expected at least 5 bytes, got %d", data, len(data))
+	// Verify we have at least 4 bytes for the exponent. The GOB encoded value
+	// may be empty.
+	if len(data) < 4 {
+		return fmt.Errorf("error decoding binary %v: expected at least 4 bytes, got %d", data, len(data))
 	}
 
 	// Extract the exponent

decimal_test.go

@@ -3002,6 +3002,25 @@ func TestBinary(t *testing.T) {
 	}
 }
 
+func TestBinary_Zero(t *testing.T) {
+	var d1 Decimal
+
+	b, err := d1.MarshalBinary()
+	if err != nil {
+		t.Fatalf("error marshalling %v to binary: %v", d1, err)
+	}
+
+	var d2 Decimal
+	err = (&d2).UnmarshalBinary(b)
+	if err != nil {
+		t.Errorf("error unmarshalling from binary: %v", err)
+	}
+
+	if !d1.Equals(d2) {
+		t.Errorf("expected %v when restoring, got %v", d1, d2)
+	}
+}
+
 func TestBinary_DataTooShort(t *testing.T) {
 	var d Decimal