Insert worked incorrect as well. RocksDB used table->record[0] internally to store some
intermediate results for key conversion, during index searching among other operations.
So table->record[0] is spoiled during ha_rnd_index_map in ha_check_overlaps, so in turn
the broken record data was inserted.

The fix is to store RocksDB intermediate result in its own buffer instead of table->record[0].

`rocksdb` MTR suite is is checked and runs fine.
No need for additional tests. The existing overlaps.test covers the case completely.
However, I am not going to add anything related to rocksdb to suite, to keep it away
from additional dependencies.

To run tests with RocksDB engine, one can add following to engines.combinations:
[rocksdb]
plugin-load=$HA_ROCKSDB_SO
default-storage-engine=rocksdb
rocksdb