v.io/x/ref/lib/pubsub: fix racy code that breaks with go1.5.
v.io/x/ref/lib/pubsub: fix racy code that breaks with go1.5.

Change-Id: I213d13b04728c19a588cbd3bb475e45ade38fc62

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref