rlp: don't panic for nil *big.Int

All other pointer types can handle nil just fine.

rlp/encode.go

@@ -386,7 +386,12 @@ func writeUint(val reflect.Value, w *encbuf) error {
 }
 
 func writeBigIntPtr(val reflect.Value, w *encbuf) error {
-	return writeBigInt(val.Interface().(*big.Int), w)
+	ptr := val.Interface().(*big.Int)
+	if ptr == nil {
+		w.str = append(w.str, 0x80)
+		return nil
+	}
+	return writeBigInt(ptr, w)
 }
 
 func writeBigIntNoPtr(val reflect.Value, w *encbuf) error {

rlp/encode_test.go

@@ -196,6 +196,7 @@ var encTests = []encTest{
 	{val: (*uint)(nil), output: "80"},
 	{val: (*string)(nil), output: "80"},
 	{val: (*[]byte)(nil), output: "80"},
+	{val: (*big.Int)(nil), output: "80"},
 	{val: (*[]string)(nil), output: "C0"},
 	{val: (*[]interface{})(nil), output: "C0"},
 	{val: (*[]struct{ uint })(nil), output: "C0"},