[#598] Fix use-after-close bug in badger writecache

Signed-off-by: Alejandro Lopez <a.lopez@yadro.com>

pkg/local_object_storage/writecache/writecachebadger/flush.go

@@ -80,6 +80,13 @@ func (c *cache) flushSmallObjects() {
 			continue
 		}
 
+		// Using the db after Close will panic and badger won't wait for outstanding txs,
+		// so we need to check manually.
+		if c.db.IsClosed() {
+			c.modeMtx.RUnlock()
+			return
+		}
+
 		_ = c.db.View(func(tx *badger.Txn) error {
 			it := tx.NewIterator(badger.DefaultIteratorOptions)
 			defer it.Close()