Fix some build warnings under clang

Change-Id: I2a84b5c7fdc7720fd5dc2c3a5df02120b0786383
Reviewed-on: http://review.couchbase.org/28778
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>