Check producer before use in ActiveStream::backfillReceived
Check producer before use in ActiveStream::backfillReceived

With recent changes for MB-37921 we now check the privilege of the
connection when backfilling. We still need to only do so if the
producer still exists as it could be deleted before the backfill task
completes.

Change-Id: I9e77c56e53145fb7c2e8c1ae44a30bcb4370208f
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/125668
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

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