mxfdec: Parse PreviousPartition in mxf_seek_to_previous_partition()

Without this patch the demuxer can get stuck in a loop if PreviousPartition
points somewhere where there's no PartitionPack, or if klv_read_packet() syncs
back up to the current partition.

This should fix Ticket3278 properly and unbreak Ticket4040.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
