Check return value from asprintf
Check return value from asprintf

Otherwise gcc outputs warnings:
ignoring return value of asprintf, declared with attribute
warn_unused_result [-Wunused-result]

Change-Id: Ib43af5d5faf2cff527146b0f315c5146d693ed07
Reviewed-on: http://review.couchbase.org/43044
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,cloudrain21/memcached-1,daverigby/memcached,owendCB/memcached,mrkwse/memcached,daverigby/memcached,couchbase/memcached,owendCB/memcached,mrkwse/memcached,daverigby/memcached,jimwwalker/memcached,cloudrain21/memcached-1,jimwwalker/memcached,owendCB/memcached,couchbase/memcached,daverigby/kv_engine,cloudrain21/memcached-1,couchbase/memcached,mrkwse/memcached,owendCB/memcached,couchbase/memcached,jimwwalker/memcached,mrkwse/memcached,daverigby/kv_engine,cloudrain21/memcached-1,daverigby/memcached