a11y: Fix crash in treeview

This is a band-aid fix. The master branch has a complete rework of the
treeview a11y code. Let's hope this will cause most crashes to magically
disappear. This code has been indexing by random memory on the stack for
a long while and things didn't crash, so let's hope for the best.

https://bugzilla.gnome.org/show_bug.cgi?id=663322
https://bugzilla.gnome.org/show_bug.cgi?id=664137

gtk/a11y/gtktreeviewaccessible.c

@@ -3275,8 +3275,9 @@ refresh_cell_index (GtkCellAccessible *cell)
     return;
 
   cell_info_get_index (tree_view, info, &index);
+  g_hash_table_steal (accessible->cell_info_by_index, &cell->index);
   cell->index = index;
-  g_hash_table_insert (accessible->cell_info_by_index, &index, info);
+  g_hash_table_insert (accessible->cell_info_by_index, &cell->index, info);
 }
 
 static void