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.

(cherry picked from commit 4fc5d7a5f5)

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. */