db: negative unreferenced value size

[jbowens]

https://grafana.testeng.crdb.io/d/StorageAvKxELVz/storage?orgId=1&var-cluster=drt-chaos&var-node=All&from=1752147081951&to=1752187178745

Jira issue: PEBBLE-1089

[jbowens]

Maybe this is a consequence of estimating virtual sstables' references through linear interpolation. If one virtual sstable is estimated to reference X bytes but it actually references zero and another virtual sstable of the same backing file gets materialized into a physical sstable, we're now double counting values.