Fix recovering of old crashed sessions having sprites containing slices

[martincapello]

Without this fix a recovering thread could get stuck in an infinite loop when reading the slices of a sprite created with a doc format version lesser than 2.

This is something I missed when I did #3752

So this issue happens when recovering a sprite from a crashed session in Aseprite v1.3-rc2 (or later) that:

1. Was created in an Aseprite version prior to v1.3-rc2.
2. Contains slices.