fix: Fix panic in ingester.go (#13557)

Author: grobinson-grafana

pkg/ingester-rf1/ingester.go

@@ -254,6 +254,7 @@ func New(cfg Config, clientConfig client.Config,
 		instances:        map[string]*instance{},
 		store:            storage,
 		periodicConfigs:  periodConfigs,
+		flushBuffers:     make([]*bytes.Buffer, cfg.ConcurrentFlushes),
 		flushWorkersDone: sync.WaitGroup{},
 		loopQuit:         make(chan struct{}),
 		tailersQuit:      make(chan struct{}),