add {heap}.IsEmpty() and use it to empty the block during write()

day-09/internal/two/disk.go

@@ -124,7 +124,7 @@ func (d *disk) write() {
 	allBlocks = append(allBlocks, d.fileblocks...)
 
 	for i := 0; i < numEmptyBlocks; i++ {
-		for range d.emptyblocks[i].Len() {
+		for !d.emptyblocks[i].IsEmpty() {
 			allBlocks = append(allBlocks, heap.Pop(&d.emptyblocks[i]).(*block))
 		}
 	}

day-09/internal/two/heap.go

@@ -25,3 +25,7 @@ func (h *minHeap) Pop() interface{} {
 	*h = old[:n-1]
 	return x
 }
+
+func (h *minHeap) IsEmpty() bool {
+	return h.Len() == 0
+}