MB-100: fix a few compiler warnings

Change-Id: I4655cedf61b19507d2f79e7dc92632529739b5c7
Reviewed-on: http://review.couchbase.org/28826
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>