stack: Never focus unmanaging windows

We can enter weird states where get_default_window is called during
window unmanagement, before the window has been fully removed from
the stack. Make sure these windows are *never* returned from
get_default_window, as focusing them can cause an assertion fail,
or worse.

src/core/stack.c

@@ -1730,6 +1730,9 @@ get_default_focus_window (MetaStack     *stack,
       if (window->minimized)
         continue;
 
+      if (window->unmanaging)
+        continue;
+
       if (!(window->input || window->take_focus))
         continue;