fix slab clearing error

weed/util/mem/slot_pool.go

@@ -42,7 +42,7 @@ func getSlotPool(size int) (*sync.Pool, bool) {
 func Allocate(size int) []byte {
 	if pool, found := getSlotPool(size); found {
 		slab := *pool.Get().(*[]byte)
-		for i := 0; i < size; i++ {
+		for i, _ := range slab {
 			slab[i] = 0
 		}
 		return slab[:size]