Check for window_event_result::deleted for ui_get_filename polling loop

This is instead of using window_exists, which could give a false positive if a new window was allocated with the same pointer value as a deleted one.

common/ui/file.cpp

@@ -301,8 +301,7 @@ int ui_get_filename(char (&filename)[PATH_MAX], const char *const filespec, cons
 	
 	wind = ui_dialog_get_window(dlg);
 	
-	while (window_exists(wind))
-		event_process();
+	while (event_process() != window_event_result::deleted) {}
 
 	//key_flush();