Cope with "delete_window" event on the GTK config box.

That causes the config dialog to terminate with result -1, which
wasn't handled at all by the result-receiving code. So GTK PuTTY would
continue running its main loop even though it had no windows open and
wasn't ever planning to do anything.

unix/gtkmain.c

@@ -566,7 +566,7 @@ static void post_initial_config_box(void *vctx, int result)
 
     if (result > 0) {
         new_session_window(ctx.conf, ctx.geometry_string);
-    } else if (result == 0) {
+    } else {
         /* In this main(), which only runs one session in total, a
          * negative result from the initial config box means we simply
          * terminate. */