don't require window to be around at quit time

It's created on show-splash now, so it may not exist at quit time

src/main.c

@@ -199,8 +199,8 @@ on_quit (state_t *state)
   ply_trace ("hiding splash");
   if (state->boot_splash != NULL)
     ply_boot_splash_hide (state->boot_splash);
-
-  ply_window_set_mode (state->window, PLY_WINDOW_MODE_TEXT);
+  if (state->window != NULL)
+    ply_window_set_mode (state->window, PLY_WINDOW_MODE_TEXT);
   ply_trace ("exiting event loop");
   ply_event_loop_exit (state->loop, 0);
 }