Fix compile warnings

don't initialize hrtime_t with NULL and add () to group
expression.

Change-Id: I4a09a38b8af387895f0b1e71d2f5f186430ca109
Reviewed-on: http://review.couchbase.org/29067
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>