Prevent access of freed workspace in _workspace_show

The bug triggers when _workspace_show calls tree_close_internal and
old == old_focus. Ie, when the old workspace was empty and needs to be
closed but then is accessed as output_push_sticky_windows's argument:
Breakpoint 1, output_push_sticky_windows (to_focus=0x55555589c8a0) at ../../i3/src/output.c:102
102	                    con_move_to_workspace(current, visible_ws, true, false, current != to_focus->parent);
(gdb) print con_exists(to_focus)
$1 = false

The access violation can also be prevented by checking if
con_exists(old_focus) but it shouldn't be necessary: the old_focus
container can only be killed when it is an empty workspace.

With --enable-sanitizers this causes i3 to exit but with
--disable-sanitizers the access violation doesn't reliably cause a crash
and the con_move_to_workspace call continues with:
(gdb) print current != to_focus->parent
$2 = 1

Since current->type is CT_FLOATING_CON and to_focus->type is
CT_WORKSPACE, in this specific case ignore_focus would always be true.
So, in this case, passing NULL instead of old_focus to
output_push_sticky_windows doesn't change the behaviour of i3.

Fixes #3075.

src/workspace.c

@@ -459,6 +459,11 @@ static void _workspace_show(Con *workspace) {
 
             y(free);
 
+            /* Avoid calling output_push_sticky_windows later with a freed container. */
+            if (old == old_focus) {
+                old_focus = NULL;
+            }
+
             ewmh_update_number_of_desktops();
             ewmh_update_desktop_names();
             ewmh_update_desktop_viewport();