RocksDB: Ignore null ScanContext
RocksDB: Ignore null ScanContext

When destroying a ScanContext in RocksDBKVStore, skip it if it is
null. This is possible if the backfill was cancelled before a scan
context was created. Prior to this change we could dereference a null
pointer.

Change-Id: I723219389d493e993816e316e742074644429a54
Reviewed-on: http://review.couchbase.org/85587
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine