We have a report of a panic when indexing into
`BufferConflicts.block_ids` using the `old_range` from the
`ConflictsUpdated` event, indicating that the `block_ids` array can get
out of sync with the underlying `ConflictSet`. This PR adds a mitigation
so that we won't panic in this situation, as a stopgap until the bug can
be reproduced in a test and fixed at the root.

Release Notes:

- N/A