Fix segmentation fault in stream stats

The `ldms_stream_close()` race fix (3fe7765) changed the condition of
the stream-client entry (sce) such that it can contain NULL client or
stream. The stream stats code path that also go through the list must
also been modified to support it.

ldms/src/core/ldms_stream.c

@@ -1841,6 +1841,8 @@ struct ldms_stream_client_stats_tq_s *ldms_stream_client_stats_tq_get()
 		pthread_rwlock_rdlock(&s->rwlock);
 		TAILQ_FOREACH(sce, &s->client_tq, stream_client_entry) {
 			cli = sce->client;
+			if (!cli)
+				continue;
 			if (cli->is_regex)
 				continue; /* already handled above */
 			cs = ldms_stream_client_get_stats(cli);