Dispatch: Make sure mutex gets unlocked on call to Stop

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

dispatch/dispatch.go

@@ -258,12 +258,12 @@ func (d *Dispatcher) Stop() {
 		return
 	}
 	d.mtx.Lock()
+	defer d.mtx.Unlock()
 	if d.cancel == nil {
 		return
 	}
 	d.cancel()
 	d.cancel = nil
-	d.mtx.Unlock()
 
 	<-d.done
 }