Assert in nano::rocksdb::store::clear()

I hit this assert whilst running a unit test.
```
[ RUN      ] active_transactions.fork_replacement_tally
Assertion (status.ok ()) failed
int nano::rocksdb::store::clear(rocksdb::ColumnFamilyHandle*)
/home/ds/CLionProjects/work/nano/node/rocksdb/rocksdb.cpp:602
```