Relax partition assertion for non-active SBs

Relax partition assertion for non-active SBs in write_partition.

Fixes #1292 (closed)

For non-active (support/inactive) SBs at partial frame boundaries, partition type is forced to PARTITION_NONE once the boundary geometry no longer implies a split.

The assertion p == derived_partition after only_allowed_partition int write_partition is not needed for non-active SBs since BRU overrides the partition decision to PARTITION_NONE at that point. Restrict the assertion to active SBs only, consistent with the assert already applied to the normatively-forced partition branch.