You can arrange between the scrambler and descrambler a set of special code, which signal to the descrambler that the next set of data is the start of a new set of data.
The descrambler read this data set but ignors it because its of no use to it but for synchronisation only. The descrambler then read the next data set, and from it calculate the rolling code but does not treat the data set as valid data either. From the third data set onwards the descrambler then decode them using the new rolloing code set it just obtained.