Fix a race condition, the child thread could get to the first

assert before the parent carried out the assignment of the thread
id to the variable.

Detected by:	OSX

bin/varnishd/storage/storage_persistent.c

@@ -275,6 +275,7 @@ smp_thread(struct worker *wrk, void *priv)
 
 	CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
 	CAST_OBJ_NOTNULL(sc, priv, SMP_SC_MAGIC);
+	sc->thread = pthread_self();
 
 	/* First, load all the objects from all segments */
 	VTAILQ_FOREACH(sg, &sc->segments, list)
@@ -305,6 +306,7 @@ static void
 smp_open(const struct stevedore *st)
 {
 	struct smp_sc	*sc;
+	pthread_t pt;
 
 	ASSERT_CLI();
 
@@ -346,7 +348,7 @@ smp_open(const struct stevedore *st)
 	smp_new_seg(sc);
 
 	/* Start the worker silo worker thread, it will load the objects */
-	WRK_BgThread(&sc->thread, "persistence", smp_thread, sc);
+	WRK_BgThread(&pt, "persistence", smp_thread, sc);
 
 	VTAILQ_INSERT_TAIL(&silos, sc, list);
 	Lck_Unlock(&sc->mtx);