volume: fix memory leak during compaction

fix https://github.com/chrislusf/seaweedfs/issues/1222

weed/storage/needle_map/memdb_test.go

@@ -0,0 +1,23 @@
+package needle_map
+
+import (
+	"testing"
+
+	"github.com/chrislusf/seaweedfs/weed/storage/types"
+)
+
+func BenchmarkMemDb(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		nm := NewMemDb()
+
+		nid := types.NeedleId(345)
+		offset := types.Offset{
+			OffsetHigher: types.OffsetHigher{},
+			OffsetLower:  types.OffsetLower{},
+		}
+		nm.Set(nid, offset, 324)
+		nm.Close()
+	}
+
+}