help: Fix exception with invalid fragment causing bogus extra scrollbars.

Apparently, our scrollbar logic crashed with an invalid URL fragment
(hash), which resulted in initialization not completing and thus the
logic failing.

In my view the root issue here is that we're not doing a good job of
catching JavaScript exceptions in portico pages.

Fixes #15706.

static/js/portico/help.js

@@ -56,7 +56,7 @@ function render_code_sections() {
 function scrollToHash(simplebar) {
     const hash = window.location.hash;
     const scrollbar = simplebar.getScrollElement();
-    if (hash !== '') {
+    if (hash !== '' && $(hash).length > 0) {
         const position = $(hash).position().top - $(scrollbar.firstChild).position().top;
         scrollbar.scrollTop = position;
     } else {