rlp: fix nil pointer decoding

The generic pointer decoder did not advance the input position
for empty values. This can lead to strange issues and even
infinite loops.

rlp/decode.go

@@ -367,7 +367,12 @@ func makePtrDecoder(typ reflect.Type) (decoder, error) {
 	dec := func(s *Stream, val reflect.Value) (err error) {
 		_, size, err := s.Kind()
 		if err != nil || size == 0 && s.byteval == 0 {
-			val.Set(reflect.Zero(typ)) // set to nil
+			// rearm s.Kind. This is important because the input
+			// position must advance to the next value even though
+			// we don't read anything.
+			s.kind = -1
+			// set the pointer to nil.
+			val.Set(reflect.Zero(typ))
 			return err
 		}
 		newval := val

rlp/decode_test.go

@@ -39,7 +39,7 @@ func TestStreamKind(t *testing.T) {
 		s := NewStream(bytes.NewReader(unhex(test.input)))
 		kind, len, err := s.Kind()
 		if err != nil {
-			t.Errorf("test %d: Type returned error: %v", i, err)
+			t.Errorf("test %d: Kind returned error: %v", i, err)
 			continue
 		}
 		if kind != test.wantKind {
@@ -93,6 +93,23 @@ func TestStreamErrors(t *testing.T) {
 		{"C3C2010201", calls{"List", "List", "Uint", "Uint", "ListEnd", "Uint"}, EOL},
 		{"00", calls{"ListEnd"}, errNotInList},
 		{"C40102", calls{"List", "Uint", "ListEnd"}, errNotAtEOL},
+
+		// This test verifies that the input position is advanced
+		// correctly when calling Bytes for empty strings. Kind can be called
+		// any number of times in between and doesn't advance.
+		{"C3808080", calls{
+			"List",  // enter the list
+			"Bytes", // past first element
+
+			"Kind", "Kind", "Kind", // this shouldn't advance
+
+			"Bytes", // past second element
+
+			"Kind", "Kind", // can't hurt to try
+
+			"Bytes", // past final element
+			"Bytes", // this one should fail
+		}, EOL},
 	}
 
 testfor:
@@ -314,6 +331,9 @@ var decodeTests = []decodeTest{
 	{input: "C109", ptr: new(*[]uint), value: &[]uint{9}},
 	{input: "C58403030303", ptr: new(*[][]byte), value: &[][]byte{{3, 3, 3, 3}}},
 
+	// check that input position is advanced also empty values.
+	{input: "C3808005", ptr: new([]*uint), value: []*uint{nil, nil, uintp(5)}},
+
 	// pointer should be reset to nil
 	{input: "05", ptr: sharedPtr, value: uintp(5)},
 	{input: "80", ptr: sharedPtr, value: (*uint)(nil)},