Polish insignificant assertion

Coverity CID#469229

src/fellow_storage_deref.h

@@ -50,10 +50,9 @@ sfe_deref_send(struct sfe_deref *sfed)
 {
 	struct sfe_deref_mbx *mbx = &sfed->mbx;
 
-	AN(mbx->run);
-
 	assert(! VTAILQ_EMPTY(&sfed->head));
 	AZ(pthread_mutex_lock(&mbx->mtx));
+	AN(mbx->run);
 	while (! VTAILQ_EMPTY(&mbx->head))
 		AZ(pthread_cond_wait(&mbx->cond, &mbx->mtx));
 	VTAILQ_SWAP(&sfed->head, &mbx->head, objcore, lru_list);