RepairDB crashes if a column family contains only a DeleteRange tombstone

### Expected behavior

Create a new empty DB. Call DeleteRange() to delete some data in a column family (even though the DB is already empty), followed by CompactRange().

Then close the DB and call RepairDB(). RepairDB() should succeed.

### Actual behavior

RepairDB() crashes with an assertion in debug mode builds.
In release builds (with assertions disable) RepairDB() logs a number of errors about corruption and fails.

### Steps to reproduce the behavior

Unit test that reproduces the failure in f80b4009246a6c07118dc8cdafe04b1d175490a2