defer unlock in RequestListener Close method

lack of defer looks like obvious typo

Change-Id: I890688d6e277af5640d24a9a624f48d67cbd970f
Reviewed-on: http://review.couchbase.org/41797
Reviewed-by: John Liang <johliang00@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>