only do redirection when there's redirect url on luogu page (#132)

userscript.js

@@ -105,8 +105,8 @@ const fuckers = {
   leetcode: { match: 'https://leetcode.cn/link/?target', redirect: "target" },
   linkedin: { match: 'https://www.linkedin.com/safety/go?url=', redirect: "url" },
   logonews: { match: 'https://link.logonews.cn/?', redirect: "url" },
-  luogu: { match: 'https://www.luogu.com.cn/paste/', redirect: function () { window.location.href = $("#url").text() } },
-  luogu_2: { match: 'https://www.luogu.com.cn/discuss/', redirect: function () { window.location.href = $("#url").text() } },
+  luogu: { match: 'https://www.luogu.com.cn/paste/', redirect: function () { if (document.getElementById("url")) { window.location.href = $("#url").text() } } },
+  luogu_2: { match: 'https://www.luogu.com.cn/discuss/', redirect: function () { if (document.getElementById("url")) { window.location.href = $("#url").text() } } },
   mcbbs: { match: 'https://www.mcbbs.net/plugin.php?id=link_redirect&target=', redirect: "target" },
   nga: { match: 'https://nga.178.com/read.php?', redirect: function () { $("#m_posts #m_posts_c a").prop("onclick", null).off("click") } },
   nga2: { match: 'https://bbs.nga.cn/read.php?', redirect: function () { $("#m_posts #m_posts_c a").prop("onclick", null).off("click") } },