Partition: set PreviousPartition if previous partitions exist
Partition: set PreviousPartition if previous partitions exist

Avoids requiring this to be set in a second pass in the writing
process

Limecraft/ebu-libmxfpp,Limecraft/ebu-libmxfpp,Limecraft/ebu-libmxfpp,stuarthicks/libmxfpp,stuarthicks/libmxfpp,stuarthicks/libmxfpp,ebu/ebu-libmxfpp,ebu/ebu-libmxfpp,ebu/ebu-libmxfpp