[gnome-tetravex] Fix a crash.

[Arnaud B. <arnaudb src gnome org>]

commit f70d2e55eabcc9a3ecd58c5e7fe7e3dab8f8082c
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Sat Sep 21 05:58:01 2019 +0200

    Fix a crash.
    
    Introduced by Redo work.

 src/puzzle.vala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/src/puzzle.vala b/src/puzzle.vala
index cc6390c..a240c35 100644
--- a/src/puzzle.vala
+++ b/src/puzzle.vala
@@ -451,7 +451,7 @@ private class Puzzle : Object
 
     private void add_to_history (uint8 x0, uint8 y0, uint8 x1, uint8 y1)
     {
-        while (last_move_index != 0)
+        while (last_move_index > 0)
         {
             unowned Inversion? inversion = reversed_history.nth_data (0);
             if (inversion == null)
@@ -467,6 +467,7 @@ private class Puzzle : Object
 
         history_length++;
         can_undo = true;
+        can_redo = false;
     }
 
     internal void undo ()