Set the signal handler mask to empty to avoid valgrind warning

Change-Id: I9c9fd12377b759aff38c7012edea74722dee9c1f
Reviewed-on: http://review.couchbase.org/37980
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>