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