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

All other pointer types can handle nil just fine.

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"},