MB-11629: Set engineSpecific to NULL when notify_failed_conns

Context: While notifying pending connections failed

Change-Id: I5e4085794a70a068f7a95735242b1e841cc6cf8f
Reviewed-on: http://review.couchbase.org/39982
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>