fix crash on preload

src/server/torr/storage/memcache/Buffer.go

@@ -82,9 +82,12 @@ func (b *BufferPool) ReleaseBuffer(index int) {
 }
 
 func (b *BufferPool) Used() map[int]struct{} {
+	if len(b.buffs) == 0 {
+		b.mu.Lock()
+		b.mkBuffs()
+		b.mu.Unlock()
+	}
 	used := make(map[int]struct{})
-	b.mu.Lock()
-	defer b.mu.Unlock()
 	for _, b := range b.buffs {
 		if b.used {
 			used[b.pieceId] = struct{}{}