Check if log exists before updating its contents
Check if log exists before updating its contents

Context: DCP-Producer, bufferLog
This check is required because bufferLog is initialized
only when connection_buffer_size control message is sent
from the consumer and if this size is non-zero.

Change-Id: Ic2e59d9097352e728f7762738003ea875b55e860
Reviewed-on: http://review.couchbase.org/55977
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,membase/ep-engine,couchbase/ep-engine,owendCB/ep-engine,teligent-ru/ep-engine,membase/ep-engine,couchbase/ep-engine,membase/ep-engine,daverigby/ep-engine,membase/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,owendCB/ep-engine,hisundar/ep-engine,hisundar/ep-engine,couchbase/ep-engine,hisundar/ep-engine,daverigby/kv_engine,teligent-ru/ep-engine,owendCB/ep-engine,daverigby/kv_engine,teligent-ru/ep-engine,couchbase/ep-engine,daverigby/ep-engine,owendCB/ep-engine,hisundar/ep-engine,daverigby/ep-engine,daverigby/kv_engine