reftable/table: release filter on error path

reftable/table: release filter on error path

`reftable_table_refs_for_unindexed()` allocates a filtering_ref_iterator
and then calls `reftable_buf_add()` to populate its oid buffer. On
success ownership is transferred to the output iterator, but if
`reftable_buf_add()` fails, the goto-out cleanup only frees the table
iterator and walks away from both the filter allocation and the oid
buffer that `reftable_buf_add()` may have grown.

Release filter->oid and free filter alongside the existing table
iterator cleanup.

Reported by Coverity as CID 1671512 ("Resource leak").

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>