Fix failure to update me->ui when changing preferences.

This must have been introduced during a last-minute rebase, or similar
- I'm sure it worked a couple of days ago! Because midend_set_config
passed a NULL game_ui to midend_set_prefs, the latter would make up a
temporary UI and apply the changes to that. As a result, the midend's
main UI would keep the original backend preferences, and those would
also be the ones saved to the config file.

midend.c

@@ -1998,7 +1998,7 @@ const char *midend_set_config(midend *me, int which, config_item *cfg)
 	break;
 
       case CFG_PREFS:
-        midend_set_prefs(me, NULL, cfg);
+        midend_set_prefs(me, me->ui, cfg);
         break;
     }