Buffer overflow in WTUniqueIndex cursor restore

A unique index cursor restore could return a key shorter than the current key. I such a case comparing  the current key with the restored key could cause a buffer overflow.

The evergreen run reporting this memory leak:

https://evergreen.mongodb.com/task/mongodb_mongo_master_ubuntu1604_debug_aubsan_async_sharding_op_query_4e57f57add2ad51a1c8cafd3fa05ec50b6437797_18_08_06_05_51_51/0