[gnome-tetravex] Add a security against null.

[Arnaud B. <arnaudb src gnome org>]

commit 3f5feab5c1f10eb2463409ff46a1361c067c0241
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Sat Sep 21 04:15:47 2019 +0200

    Add a security against null.

 src/puzzle-view.vala | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/src/puzzle-view.vala b/src/puzzle-view.vala
index dabefbb..c78d05a 100644
--- a/src/puzzle-view.vala
+++ b/src/puzzle-view.vala
@@ -631,6 +631,9 @@ private class PuzzleView : Gtk.DrawingArea
 
     internal void release_selected_tile ()
     {
+        if (selected_tile == null)
+            return;
+
         uint8 selected_x, selected_y;
         puzzle.get_tile_location (((!) selected_tile).tile, out selected_x, out selected_y);
         move_tile_to_location ((!) selected_tile, selected_x, selected_y, animation_duration);