Don't preload <a> elements without a `href` attribute

instantpage.js

@@ -88,6 +88,10 @@ function mouseoutListener(event) {
 }
 
 function isPreloadable(linkElement) {
+  if (!linkElement.href) {
+    return
+  }
+
   if (urlToPreload == linkElement.href) {
     return
   }

test/app.js

@@ -77,6 +77,7 @@ async function requestListener(req, res) {
     content += `<a href="${req.url}#anchor" id="anchor"><span>Same-page anchor</span></a>`
     content += `<a href="/${page}?${Math.random()}" data-no-instant><span>Manually blacklisted link</span></a>`
     content += `<a href="https://www.google.com/"><span>External link</span></a>`
+    content += `<a><span>&lt;a&gt; without <code>href</code></span></a>`
 
     let footer = await fsPromises.readFile(path.resolve(__dirname, 'footer.html'))
     footer = footer.toString().replace('__HASH__', jsHash)