[#392] gc: Use defer to mark handler done

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/local_object_storage/shard/gc.go

@@ -145,8 +145,8 @@ func (gc *gc) listenEvents(ctx context.Context) {
 			h := v.handlers[i]
 
 			err := gc.workerPool.Submit(func() {
+				defer v.prevGroup.Done()
 				h(runCtx, event)
-				v.prevGroup.Done()
 			})
 			if err != nil {
 				gc.log.Warn("could not submit GC job to worker pool",