lib/storage: properly set buf capacity inside marshalMetricID

Previously it was always set to 0. In theory this could result into incorrect marshaling
of metricIDs.

The issue has been introduced in 5e4dfe50c6

lib/storage/index_db.go

@@ -443,8 +443,8 @@ func marshalMetricIDs(dst []byte, metricIDs []uint64) []byte {
 	var buf []byte
 	sh := (*reflect.SliceHeader)(unsafe.Pointer(&buf))
 	sh.Data = uintptr(unsafe.Pointer(&metricIDs[0]))
-	sh.Cap = sh.Len
-	sh.Len = 8 * len(metricIDs)
+	sh.Cap = 8 * len(metricIDs)
+	sh.Len = sh.Cap
 	dst = append(dst, buf...)
 	return dst
 }