service_timeout_cb(): Check callback pointer before calling it

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/service.c

@@ -63,7 +63,8 @@ static void service_timeout_cb(uev_t *w, void *arg, int events)
 	svc_t *svc = arg;
 
 	/* Ignore any UEV_ERROR, we're a one-shot cb so just run it. */
-	svc->timer_cb(svc);
+	if (svc->timer_cb)
+		svc->timer_cb(svc);
 }
 
 /**